You can't look at it like that, as you have no idea of inividual business overheads. plus With a flush alone you're looking at a starting price of around £400, new controls? Pipe? Valves and fittings? Filter?
Pricing varies by location, hard to say without physically looking at the job, does that include a flush? How many rads? Bg will usually be pretty steep regardless, I'd always recommend getting a local company with a good reputation around for a quote anyway. As for better or equal that depends...
The loop you talk about is the bypass section, if you look, you can see an auto bypass between the flow and where it meets the return. Looks a tricky situation tbh as the return sits so low to the floor, the best diy option in this scenario may be to fit one to the flow.
Sounds to me that your cold water is off the mains which is why it seems much more powerful than the hot. If the were both tank feed they'd be of a similar pressure. You'll always benefit from pressure by installing an unvented as long as your old pipesizing is ok you'll benefit from flow too...