I'm building a high altitude UAV (20,000ft) and am looking for a decent engine for it. My student budget unfortunately limits how much I can spend on it but I just need it to be extremely reliable. I'm not gonna be doing any aerobatics/inverted/3D flying just steady climb then a glide to land.

I was thinking 4.2 upwards since it'll be 3m(9ft) span and I'm designing for 60kg (120lb?) on take-off.
Right now I've got about 10kg Drag on climb at 100kph with way more than enough lift to lug it off the ground. I'm looking at a sachs dolmar 4.2 (AIr hobbies) that seems to be prettty cheap and I've heard decent things about it. But again I don't know everything about it.

Any recommendations?