The factory long block is good to the 18psi range from what I've seen with good tuning. Some have run upwards of 25psi of boost on them, but at that level any flaw in your tuning is going to make itself obvious really quickly in the form of a big boom!
With that said, your goals of 250hp should be easily attainable within your budget, and aftermarket internals are not necessary
I would be looking for a junkyard long block (or an entire zxt parts car/pull-out) to freshen up. ie. new gasket set with a new head gasket, new water pump, new oil pump, new timing set, and a quick repaint to make everything look nice. Valve stem seals and a valve job for the head - would also be a good time for a new bumpstick if you so chose!
Grab some of the nicer quality knock off stuff from eBay to work with (BOV, intercooler) Look around to see what people have had success with and what they haven't.
For the turbo I would look at running an hy or other holset from a dodge turbo diesel, or a garrett from a ford turbo diesel. Many members here have had quite a bit of success with the holset stuff in the past.
A stock intake with a 60mm throttle body from a 240sx should be plenty and play well with an aftermarket EMS.
Fuel would come from a set of 440cc supra injectors (or similar, there are a few options) and a pallnet fuel rail.
Control it all with Megasquirt and Spark and you're really looking at a potent combo for a "junkyard l series"!
Take a look at what jc058625 has recently done here: http://forums.hybridz.org/showthread.php?t=158525
He does have a nicer ball-bearing turbonetics turbo, but his set-up shares a lot with the idea I've suggested above.
Hopefully this gets you going in the right direction
Then again, this should be putting out more than 225-250 hp...