You mentioned a Kubota 3130. I had a 3130 and used it for the same tasks that you plan. I never loaded a dumpster with it though but my dumptruck sideboards were 6' and it dumped over those fine. Im sure you can't get into a new 3130 with loader and grading attachments for less than $15000, you are looking at more like $18000-19000. Unless you find them used, you will be paying well over $1000 for just the attachments to grade, level and clear. Im talking a nice box blade and landscaping rake. Buy heavy duty attachments for the 3130 (or any tractor that size). Thats enough tractor to bend up the cheap ones in a second. Also towing a tractor/loader/and implements that size with 1/2 ton is a little much.
Diesel is the way to go. If you are paying cash for it, you would be fine with a used one. If you are financing, you may be better taking advantage of 0% on a new one.
Taking into consideration the use you describe, the fact that you have a price limit set and you want good parts availability, I would buy a used, major brand (JD, NH, Kubota, Kioti, MF) in the 30-35 HP range. Doesnt really matter what transmission as long as its got a forward/reverse shuttle.
