higher count spoke wheels are good... but not needed until you start having some issues with the current wheelset...
a late 80's to mid 90's steel RIGID mtb is going to be a good starting point...
but first I would hop around to a few local bike shops (LBS)... my old shop offered to glance at CL posts for a thumb up/down knowing that if they got a good starting bike they would likely be back in the shop for repairs and in a year or so for a new bike... but in the end his concern was getting people on 2 wheels... a few times he recommended bikes I was selling (used) to potential customers...
if you can find a shop that seems to be more in the business of getting people into cycling rather than a salesman you've found a good shop... it can be difficult to find but they do exist and won't turn you away just because you are overweight.