I usually only see 240s, with SRs, CAs, RBs, and VGs, but there are endless possibilities. KAs, SRs, and CAs are the easiest. RB is hard but getting more common (slowly but surely), and VGs, I have only seen one.
If I were to ever do a swap other than the Basic 3, Id get a 240Z and put like an RB in there, or start researching more on domestics, and become a torque-whore <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/hehe.gif" border="0" valign="absmiddle" alt=':hehe:'> .
For whoever posted the thread, it all depends on how much time and money you have. Getting an engine isn't much of a problem, but putting one in, especially something rare, requires lots of time, money, backup money, parts money, another car, knowledge about the engine you're getting, and did I forget money.
The problem is you said cost effective. A cost effective swap, would be CA or KA. CA seems like it would be the cheapest way to get turbo. KA would be cheapest swap, but easiest/cheapest way to get 250 from a KA is turbo. Not to sure about the CA, and how hard it is to get to 250HP. An SR, shouldn't be too hard, a new turbo and a few other essentials, and fmic can get you around that area. Or if you are a high booster you can do it on the stock engine, but better to be safe than sorry. If you get an RB20 then you'll be at 250 (I think, not sure on the hp of the RB20, only the RB26).
Good luck to your project