Uk parts support is very good for Mugen. I have been racing the MBX7R Eco since last summer, week in week out, and while I have had some MAJOR crashes I have only needed to replace 1 front lower arm. :-)
I have never had any problems getting spare/option parts and neither has anyone else I know who runs a Mugen. RPRC have most parts in stock or are able to get them within a day or 2. Herts model shop (among others) also sell/support Mugen in the UK.
Edit:
PS, they are teh sexeh.