Usually if you are leaking oil past the seals/guides, it will blue smoke at start up and then cease smoking.
Seal replacement would not be a bad idea if you are doing it yourself. That way if it doesn't work, you will not be out very much money.
I agree with the other poster who suspected something else causing the smoke. Is it BLUE smoke?
If I were going to spend over $500, I would go ahead and do a valve job and definitely replace the guides. I would do a compression, or better yet, a leak down test to determine the condition of the engine in particularly the condition of the rings and cylinders before doing the valve job. It still could be something else causing the smoke.