Ask yourself this, do you think it will be quality for that money? These kits are lucky dip you might get an ok one but most of the time they are far from it, but maybe if you bought one and sent it to a tuner then they might be able to sort out the porting, etc. but by the time you have done that you might as well just bought yourself a quality kit in the 1st place.
There are plenty of decent kits out there, the GT and Mugello kits would fit the bill but you are talking around the £500 quid mark plus a decent crank, carb, pipe, etc. Or for a bit less money then the Casa 185 kit would work well
http://www.riminilambrettacentre.com/en/casa-lambretta/kit-185.htmlAlso you mentioned you have fitted an expansion exhaust, which pipe is it exactly? Expansions don't really with low down torque which you need for tackling hills, etc. I would have thought a clubman would fit the bill better, especially with a 22mm carb. Also what gearing are you running?