Replacing Risers and Exhaust Manifolds

[bigddaddy]

I'm looking to purchase a 1987 V-20 I/O with a chevy 305 V8 and mercruiser outdrive. The owner has indicated that the risers and exhaust manifolds are in need of replacement. The boat is currently at a marina and they have quoted the work at around $1000. I am a moderate DIY'er and would like to do the work myself if I proceed with the purchase.

How difficult would this job be? How much could I expect to spend on the parts? Any caveats?

TIA for any advice.