Most of the people at Penn are doing the program full-time as well, even though the classes are at night.
There is no reason you need to take the classes at a formal program. There are numerous schools in Philly which will prepare you just as well.
I started at Penn, didn't like it, and finished up at Temple.
The thing with Penn, is that most of the teachers aren't Penn faculty, so there is a pretty wide variety in the quality of instruction.
Just go to any 4 year school that works for you as far as schedule, cost, etc. For advising, just come on SDN. You'll probably get better info here than from half the pre-med advisors out there.
As far as, postbacs getting you into a top med school.
That is one of the biggest myths out there. The students get themselves into those schools. The top postbacs are very selective in who they take, therefore, many get into top schools. Med schools really don't care where you take the prereqs (just don't go to a CC if you can help it).