John
I think you'll be very happy with the G0MRF 5 W amplifier. I bought one as a quick way of getting a few watts from my TS-890S or TS-590S on 630 m. I ran it as a QRP final (5W) for a few weeks and found it to be quite clean, and tolerant of mismatches. With 5 W I had many WSPR spots from New Zealand, which is more than 5000 km from Perth. David's kit was complete and, knowing that I was going to push it pretty hard for a while, I took care to properly heatsink the final transistor to a diecast box. I've not found any issue with overheating.
These days the 5 W amplifier is heavily padded back in gain to drive my homebrew 1 FET linear (100 W). The higher power of course gets far more hits and has allowed some JT9 two-way QSOs. I'm still evaluating my 630 m system and the band itself and, over summer, I'll either package up the existing system or do a higher power amplifier driven directly from the ~0 dBm "DRV" output from the Kenwood transceivers. (David does also sell a 300 W non-linear kit which looks well thought out. It does however pre-suppose a doubled-frequency drive which is a touch inelegant from my starting point).
Good luck with your 630 m station. (I've put a few details of my entry-level system on my QRZ page if you happen to be interested).
73, Peter.