I don't mind getting more involved if its more people you need!
I would say that increasing the contribution to £15+ would be a good idea! It's not a lot of money as far as I'm concerned for helping out with the running of the site!
Maybe the idea of only allowing purple members to be able to see a for sale advert would be a good incentive, but maybe 7days is too long?!
I kind if disagree with the oldskoolford stickers. It is a web address so I'm not really sure how you would spice that up?! I would have thought that there is more people that would rather have some simple lettering on my car then a logo, as it wouldn't suit it's style! Just my opinion! We have a dedicated sticker guy at my work with all the equipment. I should be able to get a good discount so maybe it would save some money?!
It would be good if there are more regular meets going on! I for one used to enjoy the one in Esher! I would be happy trying to set something up around this area (Epsom/Wallington/Sutton/Croydon) and maybe the meets section of the forum could have a sticky more dedicated to listing where and when the meets are and what areas so its easier to see what's going on?!
Just some of my ideas, I know it's not much but I will keep thinking!