I'd hazard to say the AAP-01 as they're under £100 stock, but they have a tendency to eventually explode unless you buy some reinforced parts for it that will take it over £100 easily. That being said, mine has yet to do so...
Outside of that, I'd say an ASG or STTI mk23 plus upgrades. Out of the box they're absolutely awful (mine struggled to hit 25m and wouldn't even hop .25s), but with a few parts they become absolute beasts (mine now hits 60m accurately with .32s, which is stupid). ASG mk23 is about £60, plus new Maple Leaf autobot hop rubber (£10), hadron TDC (£15) and hadron H-arm (£10). £95 and will outshoot most rifles, though if you want to use the suppressor that comes with it you'll need to take it apart and take out the inner barrel extension or you'll have a pocket DMR; they're about 1.5J+ with that extension because joule creep is a big thing with gas guns. However, you can run it without the suppressor no problem at all and it's still fairly quiet.
Outside of that... Second hand TM will be your best bet. I would never touch a WE as while I know some people have had success with them, the vast majority of stories I hear are horror stories with those.
My honest opinion to most though is if you're playing normally (as in, not sniping or using something with an MED), just take the hit and buy a stock TM and do absolutely nothing to it. They're fantastic pistols and will do everything you need them to do out of the box unless you're using a sniper/DMR and need a secondary that can basically be a primary.