30-35 degrees seems a good area for an all-around down hill trike. Shorter wheelbase and less rake make for good 360's, but they're less stable and not good for speed.
Cart wheels are awesome, but pricey... The inexpensive route is some cheap harbor freight pneumatic tire/wheels run in a dually setup (4wheels for 1 trike) toss the HF bearings immediately for some cart bearing.... Cart bearings are usually only $2 or so online.
Previously we've bought large pieces of PE from gas companies and had them cut down... This came out to about $40 a pair... I'll have to see if this is still the case.