Expense mostly. A B18C1 is around $3500 to buy outright. You can usually find B18B's for around $2000-$2500. That's at least $1000 you put into a turbo setup.

Depending on your confidence level of working on your car and how much you want to spend, I would decide between a B18B, B18C1 and B20 custom setup. The B18B if you just want faster and your on a tight budget. It's a solid motor and the extra displacement/torque will make a big difference. Next would be the B18C1 if you've got a little extra money to throw around. It's got the same displacement as the B18B but with the VTEC, a lot more top end performance. If you feel comfortable doing a lot of work yourself, you could consider pieceing together a B20 setup. You'll need a B20 block, a B-series transmition (preferably one from a VTEC engine), axle's, mounts, ECU and a new intake manifold. You'll also probably need a clutch and flywheel among other small things (shift linkage, wiring harness and such). Probably not something I would recommend to someone for their first swap. Seeya.