Advice on this set up

Hi, I have 1/2" copper tubing coming down the wall with a 90 degree turn with a old style valve that is frozen shut. I want to replace the valve and would like to use a compression valve. I was thinking of just cutting the copper pipe on the vertical line.

1. What do you recommend that I buy considering all the different types of valves?

2. Can I put the valve vertically or do I need to have a 90 degree bend?

3. Can you "Un-freeze" a valve?

I am a novice and bought stuff today but the guy sold me different stuff. Any help is appreciated

The best valve for toilets and other fixtures that should always be either fully on or fully off, is the 1/4 turn valve. These come in about every connection option there is, so I'd figure how to use the least number of fittings. Valves do not are what orientation they are in, but I'd try to keep the valve and supply line close to the wall as opposed to protruding out into the room. Makes for a better appearance and easier cleaning. Don't even consider spending 5 minutes on that old valve. Also use a supply line that has the braided stainless steel covering.