Rust is the biggest enemy of an Almera so check the sills, arches and front cross member. The engine should be fine as long as its been serviced regularly. Check all the fluids when viewing the car such as the oil, coolant and brake fluid.
According to parkers.co.uk used car valuation a 2000/V SRi 3 door with 90,000 miles should cost roughly £960 bought privately in good condition. So bumping it down £650 sounds reasonable. I know you said its a R reg but the oldest i could select on the used valuation was a V reg. Hope this helps.