I'm a mentor for a high school robotics team and as part of our off-season learning projects we are also building a MendelMax 1.5.
Yesterday we got most of the electronics installed and ran a test print in the "air". We have not installed the print bed yet so we just moved the z zero up a few inches and let it run. Worked fine, amazingly enough, but I shouldn't have been suprized, my kids are pretty smart.
In anycase, we'd love to chat with you about your experiences (since as you, ours is widely sourced and home built) about your experiences. It would be great to understand your print platform and which up-grades (since it seems 10 come out a day) you are pursueing.
Having built numerous 3d printers over the years including the mendelmax 1.5 I would say you should look at a self leveling bed, upgrade your belts if your using t5/ t2.5 to something like the gt2 belts or I've seen people use the cable but then again your using a printed pulley which I personally don't like if your looking for better resolution. Now if you have unlimited funds and looking the get the best printer then I would say well that's if your using the ramps board would be upgrade your steppers to at least the A4988 black edition or even larger drv8825. I would also swap out you Wades or Greg extrudes for a none printed like the makergear geared stepper motors. If you have any questions feel free to email me.
The G3D driver uses the Allegro 4988 which is what the Pololu uses. The only difference is that the G3D has a 30k trimpot to help with the off timing of the motor. Let me know what you think of it when you use the trimpot as I've never used it but might look into it depending on your review. As for the bed leveling check out this link http://hackaday.com/2012/04/23/automated-bed-leveling-with-our-3d-printer/
I suppose you could make it small but then what do you plan on printing with it. Having a smaller printer means you have a smaller print area. I always seem to run out of room with my current printers so going smaller would be questionable to me. If you plan on printing really small objects I could see this but why limit yourself. I would be more than happy to help you out if you would like to build one.
To build a printer for 100GBP($153) is going to be hard to do. The cheapest i have been able to build a printer for was around $300 and thats because i was able to print all the parts off another printer i owned. The printer Billy posted is a little over your budget bud not far off.
