Some of the pitfalls with setting a budget for restoration, in my experience at least, are:
-until you have the part in question ripped out and stripped down, it can be difficult to assess its condition. Some things look fine upon casual inspection, and later turn out to be toast, which means more money need be spent. Items like the radiator and the drive train internals come to mind.
-as you restore a large portion of your rig, the parts that aren't restored start to look like crap, and you will likely be tempted/driven to bring them up to the same standard, which means more $$$
-the nickel-and-diming, like so many above have mentioned. I would have bought a pallet load of crushed glass to media blast with at the start, and saved some money, not to mention 20-30 trips, had I known that I would need so much.
-some things cost way more than you might expect. For me it was the cos of quality paint and body materials.
-you may need to buy tools you didn't expect to fix the things you didn't expect you needed to fix.
-if you're going to buy a bunch of stuff from SOR, join their buyer's club right away, as this will save you a bundle in the long term. Wish I'd done this...
-you will waste money, in all likelihood, buying things that don't live up to expectation when you receive them. Ebay might result in a rip-off, or a vendor might send you a wrong part that still fits and cause another problem down the line, etc, or doesn't fit and needs to be sent back..
-living in Canada, the taxes and duties slapped on parts I imported from the US, Brazil, Australia and Japan really adds up after a while.
-then there's the social costs -

leaves you, friends find your lack of availability and endless mutterings about your 'cruiser unappealing after while, you turn to drink after the 9th attempt at setting up the diff., etc..(not speaking for myself here, but it has been known to happen)
You may get a good chunk of the way through, and realize you're not into it any more, you might feel like giving up, and yet ya gotta keep going, and spending money, to finish it up, or you'll end up with a pile of parts filling up the garage, or worse, rotting out in the rain, a large portion of your investment wasted possibly. It can be hard to keep going at times during a long rebuild, and ya gotta be ready for the inevitable setbacks. Determination is everything, or really deep pockets and someone knowledgeable to whom the project can be farmed out
Go for it, I say! You'll learn heaps, and the price of the education is part or the deal.