I am one that has used the iDuino. It is much smaller, but I have noticed that more than one iDuino user (me included) notices jumpy speed (and thus jumpy MPG) readings. I think the best way to go would be the kit as said above, but you may want to consider putting a couple
female headers between the board and the display, in case your soldering skills are such that you may wind up retouching your joints (I did so last night). It will add some thickness and some cost (a whopping $0.90), but you shouldn't have a problem fitting it inside an enclosure, and if you do wind up needing to re-touch some joints, or make a tweak in the future, that will be some money well spent.
I built my MPGuino and used the female headers to make my iDuino easy to detach for other possible ventures. I had to sand down the ends of the female headers to make them fit, not too hard to do. I have had it in and out several times, so I would say it has been well worth the money.