congrats, you will love this truck.
For the body- if the body is listed as fitting the Losi SCTE, it will fit the Tekno. I know there is some crossover whether the body fits a Slash or SCTE or whatever, so its hard to know without knowing exactly what body you are looking for.
Servo horn is about it. The Air Control Guards are an option to help control the parachute effect. Some like them, some don't. I think it depends on lot on the body you have and the jumps at the track.
For the wheels- you want SCTE offset which is 0. Wheels that are listed as fitting the SCTE will fit this truck.
For motor KV- popular motors are the Tekin Pro4HD 4300KV, Pro4 4600KV, (some like the 4000KV as well), a couple have tried the SMC 4 Pole 4500KV. There a lots of others I am forgetting. 4 pole motors will have better torque which modern SCT's need.
If you are running 2S batteries, somewhere in the 4XXX range will be good. Gearing will be track and KV dependent so without knowing exactly motor you have, I would be hard pressed to tell you a gearing set up. I know I have a Pro4HD 4300 and use a 18t.
Good luck, have fun. Hope this helps.