Hi Sean!
I started with an FT-817. I now have a KX2 and an MTR3B (for backpacking trips). I wouldn’t go back to the FT-817. It was an ok radio, but the KX2 is a superior rig. I am primarily a CW op, so keep that in mind. The 817 does bring 440/2m/6m to the table, so if those bands are important to you that might be a factor. I always carry an APRS capable H/T for spotting and 2m/440 FM contacts.
The KX2 isn’t as fragile as you might think. Mine has been on about 100 summits now, and is holding up fine. It has been wet a number of times, and I’ve had to remove the display cover to get dirt out of it a couple of times. The tuner is fantastic, the receiver is amazing, and the filtering is really nice. You don’t have to use the Elecraft mic - you can modify a cheap H/T mic and it will work. I’ve seen threads on this reflector with details.
You will be successful with either radio, so you just need to decide what your priorities are.
You asked about antennas. I’ve mostly used inverted V dipole antennas that don’t require a tuner - important for my MTR3B. My current favorite is a 20/30m linked dipole. Works great and cost me a couple bucks to make. Support the center with a tree or one of those cheap 20’ fishing poles from eBay - I think mine was $12 shipped and packs down small enough to fit inside my pack.
A lot of people also use various end fed antennas. There are several designs, and they all work quite well. They are easier to set up on some summits, and generally work as well as a dipole. Build a couple of each and try them out - you will find that when properly constructed they all work very well and you will have no trouble making contacts. Biggest advantage with end fed + KX2 tuner - no fooling around with links on the dipole to change bands.
Very important: make sure you post alerts before you go on a summit. When you get to the summit, make sure you spot yourself. If you get spotted you are very likely to make enough contacts. There are lots of ways to get spotted, and you should become familiar with them. SMS (requires prior set up), APRS, RBN Hole (CW only) are a few that I use frequently. I’d say that APRS has saved my bacon on a growing number of summits and is well worth taking along.
Good luck - whatever you get you will have a blast. SOTA is the MOST fun I’ve had in my 20 years of ham radio. But beware, you get high and its very addictive! =)
-Josh WU7H