Hi - I clicked on the link, but the webpage says the advert is no longer available, hence I can't offer any advice on the price as I can't see the car or what price it was for sale at. I also can't see the second car as I'm not on Facebook, but if it's for thousands cheaper, there's usually a good reason for this (I'm guessing a Cat N or Cat S car, which is something I would avoid).
In terms of the oil being very dark brown after 4k miles from the last service, I'm not a mechanic and can't speak conclusively on this. However, based on my experience, I wouldn't be surprised that the oil is no longer clear after that mileage and has darkened (although I will bow to those with more expertise on this than me).
Turning to the matter of the warranty, there are lots of things to consider, such as the condition/age/mileage of the car, how much money one has, the company offering the warranty and its written provisions. Personally, I am a bit sceptical about warranties for older used cars if for no other reason that the companies that operate these usually have lots of exemption clauses etc. However, others on this website may have different views and experiences. I would;- (1) check the warranty details thoroughly to see what exemptions there are and assess these against the age, mileage & condition of the car; and (2) look at the dealer selling the car and look for online reviews to see whether they have a record of lots of dissatisfied customers/warranty problems.
You also refer to having seen lots of Jazz cars with problems, although it is not clear if these are major or minor. However, as a general rule, I would not purchase any car if I found a problem, unless I was satisfied that I or the seller could fix it properly and within budget.
The one firm piece of advice I would give you is to not buy a car simply because you you are running out of time with your hire car. I did this when I was much younger and purchased a used car that broke down just over a year later with the engine destroyed (which resulted in me spending even more money).
I hope this helps.