Will it be pricey to service: Yes, the servicing cost as a percentage of the value of the watch is higher on these than almost any other common Omega.

Its a piggyback chronograph and the chronograph module is not typically serviced, but rather gets sent back to Omega and replaced with a new one, which adds to the cost.

If it has been recently serviced 950 quid for it is not unreasonable, if its 25 years old and has no service history you could be up for another big bill in very short order, and at that stage you'll have paid more for your reduced than you would have paid for a nice Pro version.

 

Its not that they fail its just that all watches need regular servicing every 5 years or so, if its gone longer than that then the lubricants are unlikely to still be protecting the movement and damage can occur to parts. Eventually the watch will stop when it can no longer overcome the increased drag of the lack of lubrication and wear on parts.

When you service a reduced, its not cheap, so unless the seller can tell you WHEN it was last serviced you have to assume its either "not for a VERY long time" or "Probably never" in which case its overdue for servicing already.

Whenever buying a second hand watch like this you have to factor in the service history and cost of having one performed, on a 7750 based watch like the Speedmaster Date models servicing is very cheap and easy by comparison.

 

A couple of threads where its mentioned:

https://omegaforums.net/threads/speedmaster-reduced-3510-50-or-speedmaster-3511-50.11447/

And particularly this explanation from a watchmaker's perspective by @Archer:

https://omegaforums.net/threads/need-help-buying-a-speedmaster.5667/#post-66150