Whatever you budget, factor in at least 10% more for the little crap that we all forget about...until you need it...LOL.
I was going to say 30%, but looking back I would say 50 to 100% more than you expected. There are sooo many things you don't think of. And there are parts that don't work so you have to get the right stuff.
It isn't just the engine, it is everything else too. Exhaust, headers that are supposed to fit but don't until the third set you get, then you need a higher stall convertor cuz your cam is wild.
Then you smoke your transmission because your tranny needed a cooler because the convertor slips all the time because the rear end has stock (super tall) gears and you should have got a add-on transmission cooler but it probably would have gone out anyway since the tv cable wasn't adjusted wrong.
So you get it all together and you break the stock spider gears showing off, and when the car is in the shop you notice that old radiator is leaking.
Point is there is a lot to plan for, and it all costs money.
My advice is to build the strongest and biggest bottom end you can, and use a budget top end right away. When more money comes in later, you can invest in a great set of heads and intake, etc.
Do the top end upgrade all at once if you can, so you dont have a mismatch of parts that don't act well together.
Vortec heads aren't much more expensive the stock crap, and the resale on them, along with the vortec-specific intake, is pretty good.
My biggest regret on my first build: I did a 305 as a compromise. I wanted great power and mileage. And I did get both. But with a 350, I could have gotten simular mileage, and 40 more horsepower, for less money.
And worse, I passed on buying a great running 400 sbc because it cost $350 when 350s were $100-$150.
I should have slapped on an intake and new bearings, and added some wicked parts later. I could have put the initial money i saved into something important.