I am starting to restore my mustang and need to replace the battery apron and a couple of my floorpans.  One of my neighbors is selling me a Lincoln Electric pro core 100, is this good enough to do the job?  

I believe that is a flux core welder... what you need is a mig machine with a solenoid for shielding gas... like the Lincoln SP-135T.

Flux core welding uses no shielding gas. Its sort of like stick welding with a wire. I would suggest for the sake of a sound weld, the SP135 or SP175 (220V)... both decent and make a nice clean weld...
There is a conversion kit for the pro core 100... its a about $140
dekrev  

I've been using a lincoln 140c 120v,can be used with flux core or gas shielded mig. inexpensive yet high quality with variable voltage adjustment. I was new to welding and I'm getting very good results with the welder  

I consulted with a body repair guy when I bought mine because I figured they would know what to use. He recommended and I bought a Miller 135 that works on 110v. Great MIG machine. A little more expensive then the Lincoln which is also great. You can find either on ebay,,used.