That is a loaded question as the prices have increased a LOT. I purchased 5 20" rims from Buckstop @ $2935 and sourced the MPT 81's from a local commercial tire shop I had worked with before. I have looked into replacement MPT's and they seem to run from $500 to $650 each. Good news is we only need 3 or maybe 4. I have it on good authority a new compound MPT should be out sometime soon. Not sure what Buckstop is charging for their steel wheels today. There are good aftermarket aluminum rims out there, not cheap but VERY well made. I was able to pick up my Buckstop order on the way home from O.A.T., so saved a bunch on shipping.
As for super singles, I like them. You could stay dually and drive it most places a super single can go. YES, the super singles have some advantages...SOME! I would have LOVED a SRW 37" non military tire that could take the weight of our rig, yours possibly being lighter could use the TOYO, or something similar with a higher load rating. I would have no problem going dually again if NEEDED.
There are a LOT of options the 5500 flatbed camper guys are using. Good FB group as they discuss this in detail, DRW and SRW.
FYI I love the Keldeman, the ride is a LOT!!!! better. Takes wear and tear off the truck and box. I can make repairs and make air anywhere. I looked at the Liquid Spring, lots of parts and special fluid. IN MY CASE...having lots of experience with air ride I decided on air. I know Liquid Spring owners who are very happy with their systems. PERSONAL choice.
I did ALL of the installation on the Kelderman and the Super Single conversion myself. Not hard, just time consuming.
Our 84' CA is just about perfect for the 2 of us, and our layout. Our subframe is 18" longer than the frame and my truck frame mounted custom bumper is made to fit. FOR US a 60" CA was just not long enough, it might fit your needs perfectly! A 120" was WAY too big for us. We spent a TON of time planning! SO I GET IT!
CHEERS!!