Thing is, not many 4X specific frames made nowadays but for that budget you could track a luxury second hand frame like Yeti, Stanton etc old dialed or something like a DMR Omen Or you could go 650 an get a small hardcore handrail. Plenty of them about
Yeah 7 is good. Honestly you're at most only running through 2-4 on a track. I know graves put out a recent post about his WC yeti and the gear setup on that. He was saying he only really used at most 3, but most times 1-2, but had to put 5 on there due to a UCI rule.
I love how sleak my Yeti is right now as a single speed.. But I keep 3 cogs on the rear wheel for DJ and then 3 different BMX tracks I race at.. I can make up the difference in the sliding dropouts..
But part of me almost wants to toss a derailleur back on. BMX racing, some of these kids can spin a 130+ rpm and I can't.. So they are able to snap the gate faster and get position in the first turn. I then spend the rest of the track catching them and eventually am about to use my taller gearing later on in the track.. Having a "gate" gear and then bang it into my "track" gear at the first jump would be super slick..
I'm running 7 on my Blur 4X and I ran 7 on my Ticket S. If you intend to use the bike for anything other than 4X/slalom then it's nice to have more gears.