Congratulations on the boat Kim, sounds like you got a steal of a deal there. Hypalon fuel bladders would be great but to buy them new is outrageously expensive. I was quote over $ 900 CAN for a single 20 liter (~6 US gallon) bladder. If you are able to find a source of used ones at a reasonable price I'd also be interested.
Have yous considered using one of the plastic 5 Liter NATO jerry cans made by scepter? With the universal adapter cap kit they make, you can draw straight from the jerry can to the outboard? I carry 4 of these on my boat and they work fantastically - far, far safer and much more space efficient than the typical outboard fuel tanks that are out there.
As for engine options, I thing twin 50 mercs would be way too heavy for a 5.3 m boat. Even if you were able to get a pair of the lightest weight 50 hps (older Tohatsu/Nissan carbureted 2strokes that are no longer offered in Norht America or Europe), you'd still be well over 300 lbs. on the back. I'd say go with a single 50-65 hp outboard, then have a ~6 hp outboard that weighs next to nothing for trolling and as an emergency kicker to get you to a shoreline.
While very pricey, think that the cat's meow for your boat ( and for mine) would be a BRP 65 ETEC commercial with the tiller steering & rope start. They weigh somewhere around 230 lbs and apparently they are amazingly fuel efficient. I typically take 80 liters of fuel ( 4 NATO jerry cans) with me for most outings and often find that to give a pretty limited range (just over 200 km/ 120 miles) between refuelling stops with my carbureted 2 stroke Yamaha 40. The 65 hp ETEC would no doubt give a considerably longer range with that same amount of fuel, give way more top speed, be much quieter and reduce the emissions that I inhale considerably.