ybajwa- Welcome to Autopia!
The undercar rust issue sorta depends on how involved you want to get.
Very involved: clean and degrease, treat rusty areas with rust remover (I've been using OxiSolve from
Eastwood ), then coat with either the POR15 or Eastwood's Rust Encapsulator (which I like a lot better, having used both). Then you can top the Rust Encapsulator with paint if you like. Hit up the appropriate areas with an undercoating (including spraying it in the frame rails).
Much less involved: clean it up fairly well and *only* spray it with some kind of undercoating. I discussed the various types on a recent thread that might be worth searching for. I'd get the undercoating from either Eastwood or
AutoInt.
Most of the time I'd recommend just using the undercoating approach. Then spray it down at washes and touch up the undercoating as needed. I'd use the involved approach if the existing rust is pretty limited and you're confident you'll be able to keep things clean after you fix it up.
Products like Griot's
Undercarriage Spray work pretty well on undercarriages in fairly good condition.
When spraying it off, you might try this (which is what I do at every wash). Rinse it down, then spray on an APC (I usually use EF High Intensity). Let it dwell and brush it with the appropriate brush (I do this lying on a creeper and I usually jack up the vehicle to gain access). Then rinse it off. This works great for me, but most of my vehicles have been pampered since new so their undercarriages are still pretty decent.