Low compression on two cylinders

[95Blue240sx]

Edit: make that low compression on all 4

My GF's car(96 s14) broke down the other day. I had a friend check it out since I was out of town. Ends up there was no coolant and it was low on oil. She said that the car just stopped responding so she had to pull over. It cranks over but doesnt start. Long story short the car is back at her place and I just check compression.

120
120
70
80

That is what it came out to, the first is cylinder 1 for reference. When I took out the spark plugs for to check compression, the last two were greenish and smelled just like coolant.

At this point I know the Head Gasket needs to be replaced, and I will get the head resurfaced. When I try to start the car it cranks over and will catch and sound like it will start for a second the just keeps on cranking. I talked to a few others and they told me it was just the HG. I am posting because I dont want to do all the work and find out I have other problems.

I plan to get a Head gasket set and will get the head resurfaced. I will change the thermostat as well. Will there be anything else I will need to do? Will I need to get new headbolts? How hard would it be to change the head by myself? I think I might have a replacement head on standby so I can just swap them out. Can it be done in one day if I have the replacement head?

thanks.