If you look at the Studio Profile in Feb PSN you will see a high-end studio project we did in NY called KMA Music.. We used the exact concept you are considering albeit on a larger scale. The isolation shells were all pre-fab construction (IAC Moduline panels, floors and ceilings for this project). We do a lot of work with IAC Doors, and some projects using their modular shells for isolation solutions. The main reasons we use them is typically financial (they can be leased as a fixture... not permanent construction), or for the reason you list: wanting to be able to take it and relocate.
It is possible, but I have to agree with Brett on the size issue. This is a very small room you're talking about.. Remember all the same issues with small rooms apply to pre-fabs, so be careful about your room proportions (use one of the calculators that are available) and expect to have to treat this room, install electricity and air conditioning.
The IAC modular rooms are not cheap! It is typically more expensive to go this way vs. traditional construction, but the advantages are as listed above.
I would strongly recommend you go for the largest room you can afford and hold some serious budget for the treatments, electrical and HVAC work. Also, make sure you are specifying the performance you need: STC values, etc. Don't expect an inexpensive booth from the 'back of Mix' to be a panacea. It won't be... BUT, you can do what you're talking about, I assure you, from personal experience.
FM