The how tough is Harris is part of the reason I’d go as easy/cheap as possible. You have no idea what the right bike is. The question about new vs old, or fs vs HTC can be answered with demo days or hire bikes. I think that’s better done with q recent memory to compare with though.
FWIW I’m on a really short by todays standards bike. It is sort of slack though. The point is that confidence is built. If I try and ride it on stuff way outside my comfort zone, that’s bone breaking territory. If I push the zone just a bit, it’s fun, safe and rewarding.
I probably give up a fair amount of speed and ride stuff way easier than I would on a longer bike but that’s OK. It’s the same as the fs vs ht argument or anything else really. At some point you’ll want something different to progress, but that progression could easily be going the other way to a gravel bike.