The biggest batch that you will be able to yield with a converted keg is 10 gallon.
If you want to do 10 gallons or less then this is the best way to go in my opinion.
A converted keggle is made of 316 SS (better than 304) and is double as thick as most other SS kettles.
Keggles are not ideal mash tuns due to their lack of insulation and small base.
But, I do get the same efficiently with a with 11lbs of grain as I do 25lbs of grain.
Perhaps a 10 gallon Imperial batch may give you a lower efficiently but I haven't ever done one.
HLT doesn't need to that big. On average it takes about 16 gallons of water to do a 10 gallon batch.
But this is separated in stages; so for example you would use 6-8 gallons to strike, refill and heat, use 6-8 gallons to rinse.
I use my original SS kettle that I bought years ago for my extract batches for my HLT, it is only 7 gallons but works great.
All this said, if space is the biggest issue then you may want to take a good look at getting stock pots that can stack inside each other.
For example get a 30 gallon mash tun, 20 gallon boil kettle, and a 10 gallon HLT. Then stack them inside each other for storage.
Aluminum stock pots are pretty cheap and work fine once you season them well. SS will cost you a small fortune.