Elite don't yet do a motor mount that'll fit the Hyper ST. RC-Monster's 'Hyper' mount does fit, and there are quite a few Hyper ST conversions on their forums.
A slipperential is, as the name suggests, a 10th-style slipper clutch, combined with an 1/8th-style differential. Definately worth it with the Tekin for added peace of mind.
The Tekin system is good, though expensive, especially when you have to factor in the Hotwire. I think Stormer Hobbies were doing them really cheap, so have a look.
1700KV is a faster 5S motor, and a slower 4S one, so it's not a bad idea to start with 4S then go 5, but there isn't really much point to be honest. Just go 5S to start with, then use punch control or the slipper settings to calm it down.
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