250 WHP N/A is a bitch and your going to spend a hell of a lot more than 2-3k
I spent 2+k On my head alone two years ago. But im sure you can build a nice N/A B18a/b with 2k.
I have a JG engine dynamics Ported and polished head with EVERYTHING done to it just sitting in my closet......was gonna build a nice N/A B20 but things came up so now its on the market.
With that said and out of the way here is some pointers.
keep your Stock LS crank because it has the longest stroke from all the B-series Engines.
Also your stock crank is good up to about 3 times the power your going to make so there is no point in spending $900-2000 on a forged crank.
Now Rods are always a great thing to buy, They are what always breakes first(well rod bolts).
Buy some eagle rods as insurance, buy CTR pistons, use a OEM head gasket, A nice camshaft of your choice, and last but most important GET IT TUNED!
your compression with a LS head would be at about 11.6:1 according to this
.:FFS TechNet : B-series CR Calculator :..
The most expensive part of the build is the head and any machine work.
Good luck and I hope I helped in any way.