I wouldn't use a dual fuel system for the following reasons...
1. You'll have two pieces of equipment to maintain
2. The controls to switch between the two can fail, possibly leaving you without heat
3. Having oil tanks to maintain is a liability
4. Most of quebec's electricity comes from hydro stations, so fossil fuel costs don't significantly influence the cost of electricity. The price is likely to stay flat as the cost of oil rises (trust me, it will)
What do you mean by oversize? I cannot possibly undertake a complete house renovation all at once.
The capacity of your boiler should match heat loss of the house on the coldest night of the year with a small safety margin. If you change the boiler now and upgrade the house later, the heating system will be too large.
Oversizing results in short cycling and large temperature swings. (the problem tends to be less pronounced on a hydronic systems compared to forced air)
Before making any changes, have a whole house energy audit done. Some things such as blowing in insulation (attic, walls if framed) and draft proofing can be affordable - the grants in some cases can almost completely cover the cost of insulation upgrades
. Other upgrades such as replacing windows are expensive; plastic shrink wrap is a good temporary solution.