Liquid damaged MBP works with external keyboard/mouse - salvageable?

I was given a Macbook Pro that is liquid damaged (with what liquid, I'm not sure). It still boots up and the OS seems to run normally, but there is no response when you 'click' the trackpad (though you can move the cursor around the screen) and the keyboard has some faulty keys (volume up/down do not work, ESC key lowers volume, etc).

With a bluetooth keyboard and mouse, the computer runs normally.

I took it to a shop and had them run a diagnostic - they said that the upper case and trackpad needed replacing, and basically it wasn't worth the cost of them fixing it. They also said that replacing those two things was no guarantee that the problems would be fixed - they said it may be the logic board.

My thoughts are that if the computer works fine with an external keyboard/mouse, it's an indicator that the logic board and memory are most likely fine. Am I wrong in this assumption?

I would like to try replacing the upper case/trackpad, and probably upgrade the RAM to 8GB while I'm inside. But before I drop ~$500 on parts, I would like to know if my reasoning is sound, or if there's a more-than-likely chance I'm throwing $500 bucks down the drain.

Any advice, thoughts, information is extremely appreciated. Thanks!