I will play Devils Advocate here and remind you of the expected timeline of the 6" dif above 200whp. I think I'm sitting at a comfortable 210-215whp now and it's hanging in there... but I think 250whp would be asking to get stranded somewhere inconvenient. I'm just saying that getting yourself a AAA membership might be a wise investment. Either that or get some 165series tires.
My advice, get a low mileage 1.8 from a trusted source... '97 or older will be cheaper leave it stock. Get the FM swap kit. Cost for both: +/-$1k... or maybe way less depending on the hookup for a motor.
Get the ETD Greddy 1.8 mani http://www.etdracing.com/miata2.html
Sell off all your bandaids and the Greddy 1.6 manifold to pay for most of it. Cost: +/-$0
Get some 460cc injectors. Get an MSpnp w/MAF delete and EBC. Cost: $1k.
Turn up boost as required for 250whp.
Periodically purchase parts for mild 1.8 build to install when it blows (could be 5k miles or 5years). Cat Rods, OEM tri-coat pistons, rebuild kit w/oil-water-timingbelt. Practice headwork on your original 1.6 head and sell it... then do actual work on 1.8 (nothing fancy).
Reinstall motor and run Greddy turbo to limits of efficiency while you save for BEGi S4/GT2860/3"exhaust/550's. Sell off Greddy parts to offset cost. Then set boost as required for 300whp. Timeline to make this happen depends on money.
More money=shorter timeline.
Less money=longer timeline.