Caps to consider: as you've already figured, polypropylenes can do
very well. Mylars/polyesters are not very stable, and don't have as
high a Q as polyprops. They also show greater distortion, which may or
may not be an issue to you. C0G ceramics should work fine. Be aware
that the loop inductance will change with temperature, too, as the loop
size changes. Presumably your loaded Q won't be so high that it's a
problem: 50ppm/C over 20C is 0.1%, which wouldn't be noticable, most
likely, with a loaded Q of up to 500 or so. At 60kHz, Q=600 is only
about a 100Hz bandwidth, so I suppose you won't want a higher Q than
that anyway (assuming you could get it). I'm curious: what loaded Q
do YOU expect to get? How big is your loop going to be? What
impedance do you expect with the loop resonated?

Cheers,
Tom