The 20 should work for you, it is steel I believe, it did survive this long. Check all welds for cracks.

The wheels should have a min of 36 spokes, or you are asking for trouble. As it is used make sure the spokes are properly tensioned. Otherwise buy used BMX wheels off ebay, CL, or look in the garbage.

If you do not want a folding bike consider a BMX. They are made for abuse, and while you weigh allot more than the typical rider you will not be doing aerials.