here is a link to a good F-18 project as an example
http://www.jettengineering.com/hall/LH-F18.html
Robart, Spring-air - or even the Cermark air gear.... but definately go with air. Just a whole lot easier to configure in this particular kit.
If you are flying high altitude, you might be a little marginal with the TH75 ---- but its not impossible. Its a fairly decent engine.
The F-18 I flew years ago was on a piped OS.61FSR.... flew nice... but close to sea level.
The plane will come in around 8 lbs. Not easy to shave off pounds, but a few things here and there to get a few ounces. Use good modeling judgement.
Gear doors.... possible, but will weight more. At 6,000 ASL , extra weight is not your friend. And it would only be for the "cool" factor anyway.... would not do a great deal to increase MPH.... lots of drag elsewhere on the plane such that doors will not really come into play.