For 300, you can probably get a lightly upgraded, or stock, used TM MP5. For 300 though your probably looking at a clone(rebrand/knockoff) like a JG/Echo 1 type rifle. The majority of rifles that are used/upgraded or come with accessories (mags, battery, ect) are usually in the 400-500 range. Once your in that price range, you have a lot more options. Typically your most common AEG's out there are MP5,s M4 or variants, G36's, G3, and AK's.
I don't really know of any games that are regularily hosted in the London area, there are some but very few. Your best bet is the Kitchene/Waterloo area. However since its indoor season, games probably won't be held for several weeks.
Currently, most games are held in the GTA.
Look here in the Ontario Games Section: