Re: My first boat....
I never delt with any or researched and vovlo penta. The Mercruiser 4.3 has a good reputaion and is easy to work on. Do some research and you will pretty quick come up with some the pros and cons of both. To tell you the truth it is going to come down to how well both of the motors have been maintained. Check all the fluids before and after running, do a compression test, and do not buy either unless you take it out for an actual water test and run that thing wide open for a little bit and then just cruise it for a while. If there is nothing wrong it, the seller will not have any problem taking you out and showing you how well it runs. Also be sure to check for soft spots in the deck, check the hull for any damage. It might not be a bad idea to get it surveyed either.
Good luck and keep us posted. There is a lot great people here on iboats that are willing to help you with anything.