For your bar and stem take a look at merlin cycles depending on the length of stem you want have a look at the Truvativ Holzfeller Stem, RaceFace Evolve AM Stem and RaceFace Respond Stem. All of thes stems are about £30 and would go quite well with what an mmmbob is designed to do.
Any of the bars on the site upto £30 will do the job more than adiquately, no real reason to spend more at this point.
As for a groupset, it is hard to look past the full deore 10 speed set at merlin that would also include brakes for £260
[edit] another place to look for your groupset – cranks, bottom bracket, front and rear derailleurs chain and cassette is rose bikes, there site can be a little harder to get round but they offer fantastic prices [/edit]