By the time you modify the bike and fix the issues that are almost inevitable in any used bike, you will be approaching the cost of a new B5, or perhaps a non- current year new C5. With the warranty and the ability to go on the freeway that the older models do not have, it may be more economical to purchase new.
When purchasing a used vehicle it is too easy to focus on the immediate lower cost without factoring in long term economy. For example, borrowing $3,000 at 10% gives a payment of $97/mo for 36 months. With $3,500 down you are in new C5 territory.
Purchase a used bike for $3,500, install a carb and pipe and some repair and you are pushing $5,000. You will have no payments, but you will also have no warranty or the ability to go on the freeway, or even cruise reliably at 60 MPH.
That's OK if that's what you want, but you should really consider more than just the initial purchase cost. I have helped many people sort out their needs. If you would like to discuss this further you can call me at 1-866-VINCEMC. I'm on holiday hours, but I will be in the store Thur/Fri/Sat.