If your plans are turbo, LS is quite suited for it. There's a couple reasons to stay away from the B20b/z as well.

• The block designs betwen the B20 and B18 are very similar, but the B20 has less block webbing for strength
• A B18 can be bored out to 2.0L and still be stronger than a B20

Also, as far as VTEC and turbos go, overlap can hurt your power some, but the head designs of a VTEC engine far outweigh the disadvantages that can be compensated for with cam gears.