Depends what you have already, £350 to set up would be a bit tricky. Live rock is going to be at least £4 a kilo, and that would be from someone needing to offload it quick in a classified somewhere. You're going to need about 30-40kg I would think so that's at the very least a third of your budget.
Then don't forget the water changes and salt, if we said salt is £2 a kilo, and very roughly 33g of salt per litre and you will be doing a 25l water change a week and then the cost of the RO water, or an RO unit for home (more initial outlay). If I used my LFS supply as an example then just the initial filling a 75G with water is going to cost you approx £50 just in water and salt.
Are you planning on a sump?
If you list your current equipment people could advise on what would be possible.
If all you needed to add was a skimmer, live rock, salt, a refractometer, and RO it might be just possible.
That's assuming you have pumps, heaters, lights, test kits, powerheads, sand, containers to transport the water in.
I am setting up at the moment and almost everything has gone over budget and forgot a few things like adding the price of bulbs to the cost of the light unit!
Could you also clarify the tank volume as 250 litres is either 55 UK gal or 66 US gal?