If you just want to see if you jell with 7-strings, I would recommend buying something used that has settled and stayed relatively constant in the $400-$600 range that you could easily flip and get your money back once you decide. I guess you could do the same in the $1000-$1500 range, just less money risked. That was my route before I decided I liked it and went all in.
For specs, go for about the same you like in a 6 string guitar, not much difference really. Most notable difference will probably be the wider, thicker neck to support 7 strings.