Hey Fred,
Hard to say what "most" means, but "Mast Mounted RF Switched Preamps" do show up at a lot of stations. To your point, one might only purchase a 2 meter version, as so many birds are mode UV. I happen to use one on each antenna (FO-29 being a big driver for that). In the US, Both SSB Engineering as well as Advanced Receiver Research preamps seem to be popular. Either will hot-switch 100 watts, both can be powered via the coax (although that is a special order item for ARR). I use the SSB Engineering SP-200 and SP-70 preamp. They were installed in 1998 and have been working flawlessly for 18 years now, surviving a bunch of bad New England weather.
Perhaps some folks use diplexers, but I sure would be concerned about port-to-port isolation. After all, you are pumping perhaps 50 watts out (+47 dBm), and expecting to receive a signal that is around -120 dBm. That's a difference of 167 dB. I don't know of any diplexers that have that kind of isolation. Without adequate isolation you are going to experience RF blocking on your downlink.
As for polarity switches, keep in mind that most satellites use linear antennas, so simply using a circular polarized antenna will help minimize deep fades most of the time. But I also know from personal experience that sometimes switching from RHCP to LHCP can make a world of difference.
In round figures each step you take is about the same cost: CP Yagi, Mast Mounted Preamp, Polarization switch. Roughly $1200 for all three steps. Of course then you need to multiply that price by two (70cm and 2m sets), and toss in another $1500 for a Az/El rotor and controller, and maybe another $1000 in coax and control cables. So $5000 for a full system at new prices. Eek!
Your plan for starting with just the antennas will far exceed your expectations compared to the Elk. As long as you don't mind pulling everything down off the tower, putting those improvements in later will be just fine. Some people will skip the elevation rotor and simply mount the beams at a fixed elevation of 30 degrees
73, Bob, WB4SON