>>> I'm a big fan of this concept: take any single Lego set, and make as many Mobile Frames and stations as you can from only those pieces. This is a perfect opportunity for the AFOL to grab a new set or two to take on vacation, after all. Smaller sets may only result in a single frame, but I recently had success with making a whole company from a single set.
>>> An excellent MFZ specific review framework has been established by Mantisking on his blog the Mobile Frame Garage. I'll reproduce it here, but I suggest checking his blog out as it has heaps of information. There's a long list of good sets for MFZ on his blog too.
Can you build a frame, or frames, right away? (Yes = +5 / No = -5)
Is it below, at, or above the golden ratio? (Below = +5 / At = 0 / Above = -10) (The Golden Ratio is $0.10 per part.)
If you can't build a frame right away, or choose not to, does it have parts you should be able to put into use right away? (Yes = +5 / No = -5)
Does it have more than a handful of immediately useful parts? (Yes = +5 / No = -5)
Added to a base score of zero (0), the answers to these questions should give a range of -25 to +20 points.
Which gives us the following scores and ratings:
+20 = A+
+15 = A
+10 = A-
+5 = B+
0 = B
-5 = B-
-10 = C+
-15 = C
-25 = C-.
Now that MFZ: Alpha Bandit (spaceship combat) is nearing release, some people are adding scores for that as well. A marginal set for the original MFZ can sometimes be an excellent set for Alpha Bandit.
>>> When sourcing sets for MFZ, it's well worth looking on eBay for sellers who split off the minifigs & on-sell the rest of the kit. Since minifigs are mostly useless to us in MFZ (aside from the infamous 'butt-hands', perhaps), this leaves us all the good bits we're after & can make an otherwise marginal cost/part ratio dip below the line.
Just in case you don't know, if you enter a set number (like 40085) into the search engine of a site like Brickset.com, you can find a complete inventory of the parts included. Once you get your eye in for the parts that are normally used to make frames, you can very quickly determine if a set is worth your time or not.
Sometimes you just really need a particular part - you can now buy directly from Lego's online store (which is expensive), or through a service like Bricklink (unfortunately quite complex) or the newer site Brick Owl (which I haven't used yet). Careful shopping will net you orders under the golden ratio with the parts you need, but be warned: there'll always be something you forgot ;)
Finally, be aware that some countries have much better shipping services than others. I'm located in Australia, but due to our often horrible pricing I buy sets and parts from overseas. I've found that a lot of UK sellers have great shipping fees compared to sellers in the USA, and the service is often faster too.