A regular elf lives three times the human lifespan. This means a Dunmer over the age of 200 is very old. It is the equivalent of a human over the age of 65. Just keep that in mind, as cane-swinging action-grandpas are untypical on both Earth and Tamriel.
I've taken that into account, he'll certainly not be as fast and strong as he was in his youth, but then again he'll have lots of experience in bashing in peoples skulls over his long life, so it'll balance out a bit. Admittedly whilst it's true that action-grandpa's are reasonably rare, there are examples such as European knights which - if they didn't get killed in war or die of disease- were in pretty damn good nick well into their sixties and seventies, The same can be said for this guy, especially because he's an adept at restoration magic.
With old age comes wisdom, and coming in all the way from third era fills you up with a lot of knowledge. Remember that our characters are not all-knowing, even the scholarly types. Though the limit on knowledge is blurrier than combat skills, we nevertheless want no walking encyclopedias.
Well, he's been around and seen a lot, but no I don't plan on him being a font of knowledge... a font of old timey racial bias, Tribunal theology and some amusing war stories maybe, but he's not gunna know everything. He's more likely to mutter about how back in his day Khajiit made great winter fashion accessories than deliver expert lectures on the history of Daedric Princes.
Personality. I have no problem with a grumpy old fart, as long as is he capable of interactions and development.
Pretty much open to it, although I do plan on keeping the delightfully antiquated attitude.