I have seen Ref 110 amps going for just over $3500 to a little over $4000. $5000. is high. Browse HiFishark.com. I have owned both a VT100 and VT100 mk3. as well as the Ref 110. Both are OK amps but not exceptional. The Ref 110 is exceptional. It is also much easier to bias, has a tube hours meter and can run KT 120 tubes. KT 120 tubes will last much longer than 6550s in the Ref 110. I have owned a lot of Audio Research equipment. If I were looking for an exceptional used ARC amp, my favorites are a Classic 60, a V70, basically a balanced Classic 60. And the Ref 110. But biasing the Classic 60 / V70 is a pain. Also if you get something too old, you have to think about replacing caps and may need it gone through.
I really love the LS 25 mk2. I have compared it head to head with an LS27. I was no impressed with the LS27 and thought it not worth the upgrade. It took a Ref 5se to convince me that I needed to upgrade. Both the Ref 2 mk2 and Ref 3 are also much better than the LS25 mk2. Keep in mind that mk2 is very important as they use 6H30 tubes. I really like the Ref preamps because they use tubes in the power supply. I think this makes a big difference.
You may also want to consider a Fosgate Signature phono stage A very nice sounding phono stage. All tube and highly configurable for both MM and MC. They go for around 1200 used but can sometimes be hard to find.