Sounds like you've got the general plan down just fine.
Basically what I do is mash, draw out the first runnings into my pot, measure the amount of wort with a measuring stick marked by the gallon, then add the amount of sparge water that gets me to my pre-boil volume (for me, 6.5gal). It's much easier to just measure your first runnings then add the appropriate amount of water than to try and do some elaborate math to guess at what's needed. If you have around 5gal ready for sparge you'll always have enough, it's better to have too much than too little.
You don't really need a HLT in a really simple setup. You could use a bucket or pretty much any container that's capable of holding 5gal of water up to 200 degrees. You just need somewhere to put the sparge water while you drain the mash into your boil kettle. I don't really worry about the temperature too much, once I totally blanked and forgot to heat sparge water so just used room temp water... efficiency was just fine. So in other words, an insulated HLT isn't super important, if it cools a bit from your 180ish temp that's fine.
Personally I use one cooler and two pots... I heat my strike and sparge water in one pot and draw all the mash runnings into the second pot, then use that as my boil kettle. So basically I use a pot as a direct fired HLT, then just switch it out for the other pot. It's a great method if you can find an inexpensive pot. I just got sick of juggling one kettle around.