First off, I don't play competitively either so take with salt.
If the rest of your army is footslogging then don't bother getting any Rhinos, 3 of them will survive approximately half a turn if they're the only vehicles in your army. Even AV14 won't last long if your opponent has only one target for his melta/lance weapons.
I run Plague Marines in my footslogging army and use them primarily for taking forward and midfield objectives, and they usually survive as long as there's other, scarier things (
FMC's, heldrakes, oblits, havocs, termies, daemon allies) on the board for my opponent to shoot at. I use 1 melta squad and 2 plasma squads and they've always been points well spent. They laugh at bolter fire

. That being said beware of high strength, low
AP (Eldar, Tau) or poisoned (
DE) shooting. And don't forget that they can't go to ground, being fearless.
Use your allied guardsmen (or hordes of cheap cultists, possibly with a fearless character) for holding backfield objectives. Since you're paying heavily for Plague Marines' durability keeping them in your deployment zone out of harm's way just seems like a waste of points...
PS: Tanks are easy to scratchbuild with inexpensive styrene sheets, just google "Rhino / Leman Russ scratch build"...