Hi, The merc V6 range started with carbs in the late 1980s then moved to EFI (Electronic Fuel Injection) probably around early 90s, however the carb versions were still imported along side the EFI Engines. Although fuel injection EFI are not as fuel efficient as an Optimax. I think the EFI have now been dropped in favour of Opti's but you can still find the carb versions.
My 150 V6 Carb can do 60 LPH @ WOT were as the Optimax should be doing 35 LPH, certainly Wavehumpers does. However throttling back to a cruising speed sees 35 - 40 LPH.
If the V6 is in good condition then it could be worth buying as they have been around for a long time, props and spares are easy to come by and the technology isn't rocket science.
If you can live with the fuel consumption figures then go for it. Bear in mind that if you want to go really fast then that will need lots of petrol to create the power and it doesn't matter what type of engine it is. Even the latest Optis and 4 Stokes use lots of fuel at WOT. Throttling back to a cruising speed makes a big difference, ask Whiteshoes!
I think condition of the engine might be the key factor.