Mac is right on about knock-down assembly hearing testing booths. Here in the western US, MSR West is the most reliable provider of used hearing test booths. I know the sales VP there if you want a connection, send me a PM.

As for which purposed acoustic panels to buy, go for the deepest wedge that you can afford - this will help with managing reflections of lower frequencies than the more shallow wedges.

The absorption coefficient as a function of frequency is shown on each product's page. The 4" Wedge foam looks to have the best overall absorption. You don't need to cover all the walls with this; most critical is the one that you will 'sing into/play into', and you can always supplement with some strategically placed clothing or blankets or sleeping bags, etc. It doesn't have to look nice unless you are going to have clients coming over.

As for bass traps, you can make these on your own as well. Do at least one corner for each major dimension. You are trying to kill cavity modes, which with a 4' x 4' x ? room, you can calculate these quite precisely to predict where they will be in the frequency domain. Here's a website that will do it for you: http://www.bobgolds.com/Mode/RoomModes.htm

A sometimes used trick is to place non-energized speakers into the corners of the room - if the cones have a big surface area. You put a resistor across the leads to the speaker and voila - you have a tunable bass trap. The resistance value plays into this and where they are most effective. I'll have to do some more digging to see if I can find a reliable resource. I can also call up my college acoustics professors and see if they can point to any resources. This does work.

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