Upload & Sell: Off
It almost never makes sense to rent cameras (break-even is typically a dozen or fewer shooting days), but you can rent a lens 100 times before you would have been better off buying it. There's also nothing wrong with the D700 you already own.
I think 100 times may be pushing it a little (or a lot).
I've seen a couple of wedding photographers do a great job with a D200 and 18-200 (one had the rest of his gear stolen the day before!) and get some pretty impressive shots, though getting the shallow DOF that many people lust after is obviously more difficult. So, if you have the skills, it can be done with less than top class gear by today's standards.
My problem with renting is that while there are a few rental houses around, it takes time to order, get delivered (or pick up if you're close), use it, then the time to return it, and that's even if they have what you want in stock and available. The last three things I tried to rent were all out already and so not available.
You may also find you have an emotional problem knowing that it's a direct (rather than indirect) cost of this shoot and comes straight off the bottom line. While it may work out cheaper in the long run, it's an emotional problem that's hard to over come for some people.
I also hate shooting with new gear without having time to play with it before hand. Every lens has a different characteristic and unless you've shot with it quite a bit you may not realise any problems until after the shoot, when it's already too late.
You probably know all the following, but I'll throw it in just in case there's the odd bit you missed along the way.....
The biggest barrier to starting a business is usually not the startup cost but the ability to charge enough money to cover your ongoing costs. That's not just buying lenses, but the realistic cost of travel (not just the cost of fuel), the post processing time, the cost of computers, cameras, glass, lighting, web site, domain registration, upgraded web design if you can't do it yourself, accountancy bills (something many people forget), insurance, local taxes (where applicable) etc, and that's before you even take out any wages or pay income taxes. All this is without even considering premises to trade from.
Your home insurance may not cover working from home for money, and it certainly won't cover paying clients coming to your home and injuring themselves. You'll need additional liability insurance for all these types of things.
Don't forget you'll need to get some contracts done for your clients to sign if you don't already have them. This could mean there's some legal bills too. Don't shoot for money without T&C and/or a contract in place. It's tempting to try to find a free contract on the internet but unless it's both designed for your local region 'and' up to date with current legislation (which does change) then you could find out that the money you saved was not good value if someone decided to make a claim against you.
Our four biggest annual bills are promotion, travel, accountancy, insurance (in that order). The start up cost is tiny in comparison. Often you need to invest a lot more money in getting things running than you'll have coming in. I haven't included gear in the list because it varies from year to year.
The cost of business promotion is often totally under estimated when getting started. How will people actually find you, and once they've found you, why would they chose you over the next guy? If you compete purely on price you'll never get anything but the bottom of the barrel, which is also where the most problems come in. The less people pay the more they seem to expect.
If you've been doing jobs on the side and selling prints but still don't have the cash for more gear, let alone startup cost, how many multiples of those shoots & prints would you need to sell to be making a rational income after all the expenses are taken out? Is that number actually achievable without getting burned out, and if so, how will you find all the extra customers? Where will your break even point be, both with and without you taking a wage?
There is a myth of "build it and they will come", but in business that's just not the case. Opening a business will not make people come and spend money with you just because you are there. You provide a compelling reason for them to do that, so what will that be?
Photography is totally discretional spending for most people, so you also need to be sure that there is both demand in your area AND that people have the money to spend in the quantity you'll need. It's no good asking $5K - $10K for wedding photography (as people on Creative Live keep telling us all we need to be doing) if most people's entire wedding budget in your area is only $2K.
Many startups think of their time as free, but that's just not the case. You need to be charging at least as much and preferably more (in fact multiples of more) as you would have to pay an employee to go do the work for you, including all the time they spend in the back room (preparing gear, computer time etc) AND travelling. Then you need to add a profit margin that allows you to cover the fixed expenses (accounts, insurance etc) and to totally replace all your gear (computers, car, body and some lenses) every 3-4 years. That's not the cost of buying all new gear, but the difference between what you can sell your old gear for and what it costs to buy the new gear. While you can write some of the depreciation off against taxes, you'll need to actually be making a profit for that to even come in to play.
Once you've figured out how much you need to be charging then you can figure out if you can even get the work to justify the initial costs of startup. Remember, most business startups fail because of poor cash-flow planning and inability to service the overheads from the income. They don't realise how hard it can be to get over the first few months with insufficient income to pay the bills and they often over estimate their sales (often by 90% or more!) which then don't materialise.
If it's a hobby that you're running on the side there is a temptation to do it cheap to get started, but then you have a really long haul trying to raise prices to the point it becomes sustainable full time (if that's where you want to go). Photography as a hobby is wonderful and it's easy to get drawn in to the romance of wanting to do it for a living, but as a profession you're likely to spend less than 20% of your time shooting (often 10% or less) and that number needs to earn enough to cover the rest of the time and still allow you to buy new gear and pay your overheads.
So the cost of startup may only be $700 or so, but you need to plan for additional costs (such as promotion, travel, legal and accountants) down the line and make sure you're going to be able to cover it, otherwise you're totally wasting all that startup money in the first place.
Oops, sorry this got so long....!