Sorcha, money certainly does enter the equation but I see no reason to suspect that your machine will not give you years of reliable service - plus it sounds like you have a good service deal. It could well be a good buy and it could well suit your needs for years to come....I think all cases are different. I am a "sling it together myself" person and keep things running that way largely because of finances but I have enough ability to change bits when necessary to keep me current enough to do what I want to do..
On the other hand I suggested my mum bought a Packard Bell, simply on the grounds ofher lack of tech knowledge and the support deal she was offered from the store close to her...
Maybe you should just enjoy what you have now which is more than adequate for most users - I have a 500 and you have an 850... and I have no plans of upgradingin the forseable future.
Jon