The Konica Auto Reflex was the only production SLR with a selector for both full and half frame that I know of. The Olympus Pen F series was designed as a half frame SLR and featured a very flat body design with vertical mirror and no mirror box. The Olympus lenses for the Pen F series are compact and getting hard to source as age and attrition thin their number. Besides the Pentax KX and Nikon FM there were several regular SLR's that were custom modified to shoot half frame. Obviously very rare and expensive. If you really want to get into half frame with interchangeable lenses then Olympus Pen F is still the way to go. Buying from ebay you could probably cobble together a small system (body, 25mm f4, 38mm f1.8, 100mm f3.5) for about $1000 for clean, working samples.
The TTL meter in a Pen FT is workable (if working) but is a klugy 'transfer the reading' type. I always just carry a hand held meter for my Pen F and when I had a FT serviced, had the meter removed entirely and the beam splitter mirror replaced with a full surface one. This increases the brightness on the focusing screen half a stop.