I committed myself to coding it in AS3 to start off with, but progress has been slooooow so instead of trying to get my head around the complicated innards of the new language I decided I may aswell lay it all out in AS2 and then convert it piece by piece to AS3 when it's all done (or pay some other sucker for the task). Don't get me wrong, I'm all for AS3 but Jesus does it make the simplest tasks a pain in the arse at times, I mean, just the other day I was trying to create some simple lines to show me the co-ords of bounding box on an mc- something which would take like two secs in AS2- and three hours later I still couldn't get the bastard compiler to stop throwing out error messages. I guess I'm just an ambitious rodeo-cowboy without the chops to pin down the beast, for now.
I've decided to keep note of what changes between the two languages screwed me up so far, and will be creating a new little AS2-AS3 Ref.Guide for Game Programmers to go up on the Sidebar any day now.
As for the engine, I started off using TonyPa's Tile-Engine but being frank I think if you're going to make a game that has as much art in it as say Metal Slug you're better off with art-based, as you're not repeating many tiles anyway. Andre Michelle created a sweet little art-based prototype for mario over on GotoAndStop.it using vectors and so far it seems the best option to me, being I can derive the co-ords for every platform just by drawing a line over the art background and taking the x,y for each end, and don't have to muck around with a ton of code, either.