I want to use this space for vocals and guitar tracking...

Ok, here's the deal. I'm living in an apartment complex right now and I have been using living room to track guitars (half stack amps, acoustic, etc...) and my closet to track vocals. I am not really happy with the results I'm getting especially with the vocal tracks. An idea popped into my head that I could possibly use my 8Lx6Wx8H storage room as a guitar tracking and vocal tracking room. The door to the room is roughly 2 feet wide and is located on one of the 8 foot walls about two feet from the corner. The walls are sheet rock and the floor is cement. The walls are not even flush with the floor so I'm definately going to have to fix that. Whatever I do I cant really do any permanant structural work to the room. I can however build inside of it. Keep in mind however that this is only going to be temporary. My budget is preferrably under $300. Here are my questions: 1) Is it possible to use this room as a guitar AND vocal tracking room? 1a) What about with my budget? 2) What would need to be done to this room to get the best acoustics for both applications? 3) Soundproofing is not a problem but from time to time I will have an extreme vocalist over and will probably need to keep him as inaudible as possible. I'm not looking for getting it 100% inaudible from the outside, I'm just looking to quiet it down a little bit. What would be necessary for me to do that? Thanks guys and I hope to hear some responses soon.