$1500 for a set of doubles is much higher than I would ever spend. Even new, a set of HP100s should be around $700, maybe $150 for a manifold and $100 for bands. Check places like DGX and dive rite in scuba, I bet you can get a brand new set for under $1000. Of course, buying this stuff used makes the most sense by far. Used twinsets are all over the place, sometimes shipping is a pain, but it's worth it. Many divers have switched to sidemount in the last several years, so you'll have no problem at all finding used bands and a manifold. I still have a pair of 7" bands laying around that are gathering dust in a closet. If you go for LP85s, you can buy mine for $50 plus shipping.
I wouldn't go any bigger than the HP100s for your first set of doubles, unless you're a linebacker or a pack mule. Even if you eventually need bigger tanks on a twinset, you'll always get use out of a couple of HP100s.