Hello Chris
Hope you will enjoy driving the Fulvia Sport for many years to come. Best establish where the rear brakes are binding - support the rear axle and take the wheels off, and look and listen. If it is the calipers that have gone sticky with lying unused, they might free off themselves, but it might be worth bleeding the brake fluid at each side just to get clean fluid to the seals. However it might be that the handbrake mechanism is not releasing properly or evenly. All that might mean is that the brake shoe lining will wear away!
If the engine is not in the first flush of youth, there is no harm in using a 15W/50 grade of oil in the 1300. Preferably semi-synthetic or full synthetic. That viscosity should give the sort of figures that Tim writes about.
The radiator header tank is also the expansion tank, so keep the coolant level about .5" below the bottom of the filler neck. Try running the engine with the radiator cap off, and carefully looking at what the coolant is doing. If you are getting great gulps of air coming to the surface and out the filler neck, it might mean that the head gasket is weeping combustion into the water jacket. If however, you are getting a milky/white foam appearing in the coolant, then it is possibly the remains of some oil in the water jacket that creates cavitation at the water pump impeller. This can be purged by using rad flush or similar. Both symptoms can cause the weeping and steam from the overflow pipe. Don't be tempted to use a pressure cap with a higher than .5 bar (7psi) spring (the figure should be shown somewhere on the cap) - you could just blow the heater matrix instead!
Regards - Roddy