I got the cheap lincoln wirefeed with fluxcore wire. I only have two patches to do, so I didn't want to spend a ton of cash. I have used a mig before, but any tips on butt welding in a patch on the door skin? I want to make sure there is enough space to apply filler.

Back it up with copper - you will have a hard time with flux core - I got a n MM175 so I could do a little more than patching sheet metal.

I wouldn't butt weld with that,save alot of time,cut your patch bigger than the hole,and drill holes along the entire edge of the outer skin,insert the patch from the inside of the panel,and fill the holes with weld,grind off and then use filler,

A couple of other options. Flange your hole and cut your patch to fit the depression so you have an overlap. Then you are welding 1 thin edge and one solid piece. It's still abitch with a flux core welder. Most flux core is .035. I did see a listing for some .030 flux core. If you could hunt up a spool of that, it should run better. Other possibility is flange and fit as above but use panel bonding adhesive and just glue it in. Finish as usual.