As someone who got ripped off owning a Yak I would advise caution.
Ours was condemned as having spar corrosion to the point that the repair would have been in excess of its value. Subsequently this un-airworthy airframe was bought off us for peanuts then flown to Germany (along with one other 'suffering' the same problem). I now see both the aircraft have been flying quite happily in Germany for over two years with no sign of major work to the airframe.
Yak sales and maintenance in the UK is controlled by a cartel of two organisations. If you try to use another organisation info is fed back from the east to these guys who promptly say 'we know you approaching other places for repair'.
If your Yak is pre-1990 build then be prepared for a 'corrosion' issue to arise due to alleged poor quality of metal when the maintenance company deems it.
Operating costs......
The Yak was based upon the USSR requirement for it to be returned to the factory every 500 hours for a total rebuild. (Kept the peasants in employment). For UK permit the UK dealers have agreed a system of replacing items on a timed basis, then getting Yak Design Bureau to sign off the work every 1000 hours or so. Some components will not last much in excess of 700 hours such as undercarriage rams which we had more than a few 'overhauled' units fitted.
For a comfortable cruise you will exceed 70 lph of Avgas and 1 litre of W100 per hour. Anything less than setting 70% rpm/700 mg will feel lumpy and soggy.
The average quoted figures for an annual will be for a 'new' airframe like driving a new car. When the aircraft grows older the figures also grow. If I could be bothered to go up into the attic I could quote some figures that would make your eyes water.
The engine has a TBO of 500 hours (750 if it is factory new for the first overhaul). New/ovehauled engine prices are riising.
Absolutely great machine which I flew for 7 years ruined by money grabbing maintenance and Yak Design Bureau.