If its cheap i'd go for it. But I think you will find the R32 is in a league of its own compared to the 320si.
I think they should have made a 325si. I too have noticed the prices seem to very cheap in the UK 2nd hand market.
As the 320si is a lot cheaper than a 320 M Sport which was more expensive but more readily available. I expected the car to be least available to be more expensive.
I think a lot of the Si's were bought by dealers and then sold as ex demo cars.
Shame really as it is a good car.
In answer for your query on reliability I would not worry too much about the idling issue too much.
My car gets its fair share of cold starts and hot starts - the mileage is nearly 31k miles in a year. If you are anywhere near the German Autobahn then its the car to get - get it to 100 mph in 5th and then floor it and hear that engine sound - It sounds like a real touring car
Shame I can't do 100mph in the UK legally