The 135 units from Lincoln and Miller are perfect for the home hobbyist. I've built Cobra frames with them, and they work great. They don't have a real high duty cycle, so you can't weld at high amperages for long periods. If you plan on welding 1/4" plate often, you may wish to step up to a 175 range machine. I would always use gas shielding VS flux core. I find with the right gas you can make some very pretty welds.
I used an Esab compressorless plasma machine at SEMA last year. It was rated to cut up to 1/8" plate, and it cut it very easily. The cool thing about it was it was 110v and you didn't need an air compressor.
http://products.esabna.com/EN/home/plas ... ry_id.1957
As far as buying, I got my stuff used. I saved a ton, and the machines work fine. At trade shows and NHRA events you can get show discounts to save a bunch too.