Sounds like putting in an ac would be a new experience for you. Im not saying you cant do it, but for a first time install you will probably appreciate letting the pros handle it. Find a reputable rv repair place and let them do it. Many nuances with that that they will have the experience, tools, and work arounds to get it done. Truly in my opinion its some of the best money you can spend on labor. As for being prewired or drainage issues, give it to the shop and let them create the solution which will be done right and probably much better than you could do. And I can assure you the headaches will be less severe. All you have to do is write the check.
A few things I just picked up on in your post.
Drainage typically will be routed to a drain pipe or hose that penetrates the floor and just free drips under the trailer.
Yes you will want a drain pan under the unit on the roof to collect and rout the runn off into the drain hose/pipe assembly. You can opt for the unit to drain onto the roof of the trailer and then trickle down the outside of the trailer but minerals in the water will leave track marks on the skin.
Yes the present cut out would probably be a good choice but without knowing your layout another location might be better. The pros can do it either way. Yes they will need to build some internal bracing for the weight of the unit. You will not see this.
As for labor, this is strictly a ballpark on my part as I have forgotten the cost for mine, but I should think soewhere in the neighborhood of 500 plus the cost of the unit.
Hope that better answered some of your questions. And ye you can run the AC with a 30 amp set up.