Hi SD50 and welcome to the forum. It sounds like your insurance does the typical "try before you buy" thing and sets you up with a rental to see if a) CPAP therapy works for you and 2) that you'll work with CPAP therapy before they outlay the cash to actually buy you a machine. That is typical.
At 5-6 hours a night, you'll be over their magic 4 hours a night for compliance...assuming you use the machine the full time when you sleep.
About pushing for something other than a non-data recording (we often call them "bricks") CPAP machine, others may have better advice to give. I'd say that you should get a copy of your prescription and see what it specifies. If it is very limiting (i.e. only one pressure), then try to talk to your doctor about getting a prescription that has more flexibility (like a range or pressure, and if you have a great doc, have the prescription for "data capable")....it'll usually give you room to push/force the durable medical equipment supplier (we shorten it to "DME") for a better machine.
Your pressure of 8 cm H20 (centimeters of water) is in a fairly common range for treatment (especially compared to 8 psi, which is massive pressure in comparison and would give you no end of grief
). Depending on the machine, they may set up up with a constant pressure of 8 cm H20 or they may have a range (i.e. an auto adjusting machine with a range of 6-20). You'll find out when you get your machine which way it is set up.
Again, welcome and hopefully someone else will chime in with some key phrases and tactics to help you avoid getting stuck with a brick.