Another vote for the V3 SP if you're on a budget. I tried a 8.5, 7.5 and 6.5 yesterday in a 2wd buggy at a testing day, and you can feel the difference in nice increments.
To use a curry analogy, I went from Korma (8.5) to Jalfrezi (7.5) and Madra (6.5)
The track I race on is Silverstone buggy club's indoor track, and the conclusion I came to is that for my current level of skill the 7.5 was about right.
Racing on a budget I think these are good motors, especially for the money. The only thing I would say is they do get hot, so I don't think you can over gear them too much or use them with too much timing. But then I ran with zero timing on my car and the 6.5 was too powerful for the slipper clutch assembly given how much I had to back it off to stop the wheeling..
I think the V4 and other £60+ motors will be superior, but not massively so, likely in the area of being able to over gear and running loads of timing. So if you're in the A final at your club night, might be worth going for a £60 motor, but if like me, you are returning and finding your feet again nearer the back, it may be worth holding out a while before going for the 'big guns'.