I find it ironic that the F5 was my 1st choice (back in 2014), but I waited a day to long to pull the trigger on it and it sold on craigslist, so I found and bought a used F4. I sold the F4 6 months later, soon after I bought the F75LTD. I do have an F2 as a back-up/loaner, I have had and used both the F70 and older standard F75, yet I never used an F5. How crazy it is that I have owned almost every one of the older F series yet never owned my 1st choice, the F5.
Both the F2 & F4 run at 5.9 khz, the main difference between the two is the F4 can be manually ground balanced in all metal mode.
The F5 runs at 7.8 khz, the same as the Technetics greek series machines (Alpha, Delta, Gamma, & Omega) I prefer that frequency over the 5.9 khz of the F2 & F4, but since my primary machine is the F75 platform I wanted a simple machine as a backup/loaner machine that allows me to have a noob up and running in a few minutes, thus I keep an F2 for that purpose.
After detecting for 5 years now I still think the F5 looks great and has plenty bells and whistles by my personal opinion is the F5 was pretty much designed for those old analog guys since they prefer dials in lieu of pressure sensitive switches.
If you are serious about this hobby, buy the best you can afford.
I have bought almost all of my machines used and have never had an issue. If you are patient and stay vigilant you can find a deal. I have seen and bought F70's and older F75s for as little as $250 and F5's go for under $200.