Sub buying

[cavalierbond]

Hi , first post so go easy eh , just got to say this is THE best forum ive come across keep it up..

Ive entered the wonderful world of Rolex with a Royal paying about £600 sterling, had it for a few years now and fully enjoyed it , but a Sub is calling

Ive been all over your wonderful forum for information so im sorry in advance if i ask already answerd questions.

After speaking to a few sellers (buy the seller not the watch) and finding a friend of a friend who i reasonably trust, ive seen a Sub he has for sale 1972 5513 no papers or box but magical faded bezel (grey blue), solid bracelet (is it 93150) correct for year , untouched matt dial and hands .
Now my questions are 1 how do i know its not a watch coupled together from a few different ones ? 2. i love having paperwork etc but understand that with a watch of this age they are hard to come by , or cost much more. Could i have it serviced by Rolex so i have some paperwork?
3 its for sale at £3500ish give or take the final wrangling .. is this fair price ?

Originaly i was going for a 2000 onward model but have been told the 5513 is a better watch/investment !!

Sorry its a bit long winded but thank you for reading it
rob