Is an old mtb with replaced gear safe to buy? I believe this question to be on topic for this site, I want it to be more factual based than opinion, another similar question is this one. If its not feel free to close it.

I am looking at buying an older (10 yrs old) Giant NRS 1 MTB, it has very good reviews for the frame especially, but other parts have been replaced (it has a new front wheel, cables etc.). Should I be worried that other parts will be about to fall off, or are replacements a good thing?

EDIT: Ok so the wheel was replaced due to hitting a stump at night time, the cables were getting old, it has had plenty of action, but was well looked after. The rear sus has a tiny bit of play in the top bush (not enough to realise when riding)

It also has a leaking front fork, not a big issue (probably new seal, or not worry), but makes me wonder if it is worth it, will cost around $400 US ish, do you think it will have other issues?

cheers in advance!
With a mountain bike parts do wear out. I would be more cautious of a 10 year old bike with original parts unless it had original tires and I could still see the manufacturer 'tags' (i.e. like new) showing it had been ridden only once or twice and been a garage ornament for all but the first 2 weeks of its life.  

If the bike has been ridden regularly you will be wanting a well maintained one. This means parts replaced as they wear out or get damaged.  

I have bough several second hand MTB's - you get a better bike for you money than new. The ones I have always ended up buying have had many parts replaced, as its these bikes that the riders are using regularity and keeping well tuned and maintained.  

With respect to the edits - a leaking front fork could simply need a new seal or it could be a new fork - its a deal breaker for me.