I doubt he can Transfigure antimatter. If he can, the containment will be very hard to get right, and he would absolutely have to get it right. How do you even stop it blowing up your wand, if you have to contact the material you're Transfiguring?

Maybe Tazers! They'd work against some shields, are quite tricky to make, and if you want lots of them they're easier to buy. Other things: encrypted radios, Kevlar armour (to avoid Finite Incantem). Most things that can be bought for 5K could have been bought in Britain in the early 90s, apart from that sort of paramilitary gear. Guns are unlikely because the twins would have heard of them.