Maintaining a large stock of good quality low ESR caps will kill your margins due to the wide range of voltage ratings and form factors used.
If you want to get this right, you need to log every part used to the job ticket in a searchable database so you can make intelligent stock control decisions. If you are going down that road, you might as well insist on adding a brief fault description refined by the service tech, including component designators so it can be used to identify stock faults.
We didn't do the database thing properly and used to operate a policy of buying one part for the set and one for stock unless the part was grossly expensive. Generic dead parts were chucked in a jar by type, and when the jars got full enough, they were sorted to identify commonly used parts to stock. If it was really cheap, we'd usually go the first price break quantity if we thought it would be a common part. We'd typically put in two or three orders a week for next day delivery. If you've got the volume you can go to daily orders.