Gus:

I don't think there are any bad choices with what has been offered as suggestions. The Pentax you linked to on eBay looks very clean, and is sold with some nice glass in the kit.

I might be wrong, but I think the format (6x6, 6x7 etc.) is probably your first decision and that will guide you towards the hardware. If studio work is going to be where you use the MF equipment, I would downplay the need for interchangeable backs since the studio is a static environment, and depending on the format you are 10 or 12 exposures away from a re-load. A little different than 35mm.

A suggestion on the eBay purchase route, I would identify some likely hardware "targets" that are offered and watch (save in your profile) how the auctions progress and the price-points where things trade (if they do), some of the "buy-it-now" sellers are delusional in their offers. Another thought is to identify a likely repair facility for what you are considering in advance of a purchase, so that the inevitable CLA cost and turnaround time are known in advance. You can also learn of any common maintenance issues prior to ownership through a conversation.

I personally own a Rolleiflex 3.5F TLR and the Rolleiflex SLX series 2 (w/ 80mm and 150mm lenses), each for over 10 years, they are both great cameras in different ways. Enjoy!

FL Guy