The only items you'll need to weld are the motor mounts,master cylander mount and alternator bracket depending on what alternator you use.You don't need to redrill the crossmember or motor mount holes,you don't need to notch the frame,you don't need to modify any oil pans (use a B20)you can use stock intake and exhaust manifolds (even with the turbo).
If you want something everyone else has,(and if you like higher registration and smog hassles) get a 240.If you want something faster than a 240 or 740 (or even an 850 with some work) do the swap.If your imagining it would be fun to drive your right! You should see the suprised look on peoples faces as you blow by them,but look quick,you'll be passing FAST.