You can use the DIY stuff, it's just going to take a lot of it to build up the thickness. If you get it somewhere between 1/8"-1/4", it'll do a pretty good job of deadening the sound, maybe even better than carpet.
You can save yourself a lot with the spray-in materials if you do all the prep work and they just have to spray it on. Remove EVERYTHING from the interior, sand down the paint (a wire wheel on an angle grinder works awesome), get a roll of masking paper and enough tape so you can cover EVERYTHING in the interior and exterior you don't want spray or overspray on, and some of the special wire-tape for the edge where the liner will end. Use the wire tape to mask off where you want the liner (and cover where you don't want it, like threaded bolt and screw holes) the day before your appointment, then show up REALLY early, use the masking tape and paper to cover everything inside and whatever their tarps or whatever covers they have won't cover on the outside, drive it in, remove the seat, cover anything you needed to drive in it, and let them go to town with the spray. You can probably get yourself a professional job for just over what you'd pay for a DIY brush-on this way.