I haven't seen the advert, therefore my opinion may not take into account all facts:
Selling price is likely to be in excess of Ksh. 250MM based on 150x monthly income.
Because there are no likely comparables of similar structures (same square footage in the same location), it is likely that the seller will accept a lower bid, e.g. Ksh. 200MM. It also depends under what sort of pressure the seller is under to sell. Of course, the more the pressure the lower the price they will accept.
Using a conventional mortgage doesn't make economic sense since the mortgage amount will exceed the monthly rent. And downpayment would be in excess of Ksh. 40-50MM. Not sure how many of us can raise funds towards that goal.