Keggle question

I've been slowly accumulating equipment to move over to all-grain and I've most recently found a couple of 15 gal kegs on craigslist. One has already been converted into a brew kettle but I need a little advice on what to do with the second one. At least for the time being I'm going to stick with doing 5 gal batches so I was wondering if it would be best used for a HLT or would it be worth it to try and insulate it and make a mash tun? I guess that I should also mention that I have a 5 gallon brew pot that I've been using for extract brewing so that can also fit somewhere into the system.

My thoughts were to use the 5 gallon pot for my HLT and convert the other keggle to a MLT. My only concern is whether there would be too much dead space above my grain bed making it hard to maintain good mash temps. Anyone else have experience doing 5 gallon batches in a keggle MLT?

I would make it into a mash tun just because that allows you to mash enough grain for a 10 gallon batch, which you can boil in your other keggle. You can always heat your strike water in the mash tun and heat sparge water in your 5 gallon pot.