I can give some comments having owned just about all iterations of engine and modern and ancient Alfa, so here goes :
A modern 24V V6 eats barely 10% more than an equivalent 16V twinspark.
Problems with the V6 include being willing to service them when you buy, as previous owners tend to be hounds and darn well near kill them. Minimum first service should be a cambelt and oil change, and do the water pump as well for the hell of it. I did that every time I bought a V6, and it saved my bacon every time.
Do some pricing first.
Owning a twinspark, particularly a 16V requires a Cam Variator change as a minimum on-purchase service, for the same reason as above. Most owners who sell Alfas tend to be hounds, and completely incompetent at looking after their cars. I say 'most' because there do tend to be exceptions.
For both types of engine, it is mission critical to change oil every 10,000km's whether you like it or not. Its also mission critical to change plugs every 150,000kms, usually with a Cambelt.
To reduce fuel consumption, here are some tips, which I use and consequently amaze friends with my fuel economy.
1) turn off the aircon. I honestly wish I could remove mine !
2) windows closed as much as possible. Sunny mornings at 50 km/h are ok for open windows, and for the rest, close them ! this makes a significant energy saving on the venilation system and drag.
3) Because your car is manual, you will inevitably save money. Which is another reason I hate fuel chugging autos. Practice the Bunta Fujiwara Tofu Delivery Method (see Inital D Manga and Anime) where you imagine that you have a 75% full cup of water mounted in a cup holder on your dash. (you won't actually have a cup holder, but use your imagination). Now drive normally, and attempt not to spill the water (still using the imagination ?). Fine control means smooth driving. Smooth driving means low fuel use.
4) Use in-tank injector cleaners, as these will produce a noticeable improvement in fuel consumption.
5) And now for a critical one - engine management. With a V6 it is a HUGE temptation to thrash the thing regularly, but that is NOT what the engine was designed for. When a car wakes up in the morning, even when it lives in a garage, it will be cold. Twispark, or V6, it is NEVER I repeat NEVER safe to rev it over 3000rpm until the thermometer climbs above 40 deg. C in the morning. the car should I deally not move until at least 60s after ignition. Also, oil pressure on start up will indicate about 4 bar. The car should, after warm up, indicate 2 bar of oil pressure. When warm, if it dips way below 2 (say 1.5), its time for an oil change. Then, too, the temperature should never get above 70 in normal operation. A way to ensure this is to use glycol, as well as something like Red-Line Water Wetter, whihch is a cooling system lubricant.
You can drive the car in a spirited fashion i.e. changing gear at 4500-5500 rpm, which is the Twinspark and V6 sweet spot, but be aware that fuel consumption will be somewhat worse.
That said, my average fuel consumption is about 15l of 96 octane per week, driving all over Auckland, and most of my friends are 2 - 3x that on average. There will be weeks where I buy 20 or even 30 litres of fuel, but these are quite rare. And of course if I have a spare $20, then I will spend that on some fun driving.
I guess everyone here is advising you to go for the V6 if its something you want, and just be aware of the potential pitfall. Before buying the car, ask for details of the last cambelt change. If it was 1000km ago - sweet ! buy, and look out for the 100,000 km mark. Sticker on the windscreen will do. If not - rethink, or at least see how much the job will cost.
Happy thoughts !