It stands for 'Top Dead Centre'
I.e. rather than the usual hop lever moving forwards and backwards to adjust the hop, you mod the rifle so that you press the hop down directly from above.
It helps with consistency and more.
When it comes to vsr clones, I'd go with the cyma 701.
It's a little better quality than the well, and standard 428mm length barrel rather than the Well's 500-something.
But personally, I'd go for an action army T11.
Same design, vsr compatible, but better build quality still, a more modern, funkier look, and comes with a 90 degree trigger as standard, so it'll be more durable when you put a higher powered spring in.
Plus, it's only a bit more expensive than a clone VSR, at around £160 or so (at last check).
Couple of mine below for reference.