How much is too much rent increase?

[Gary David]

Looking at purchasing a tri-plex and one of the tenants has been at the property since 1998. (WOW!) his original lease was for $450 his current rent is $490 he has 3 bedrooms plus use of the full basement for storage (he has a lot of junk)and his workshop down there. He does cut the grass as a plus but for a 3 bedroom a fair rent in the area is $750 not including the basement use. How much can you reasonably raise the rent in a new lease agreement. I am afraid if I make it too high he will leave (which could be a good thing) but also will have to remodel the whole unit. This is a cash deal so the other rents will cover the expenses with a positive cash flow just was not ready or have cash reserves to remodel the unit right now. Any ideas or thoughts would be apricated.