You mention off-plan. There are very few new-build apartments at all in central Paris. As pag says, the purchase costs are high. If you buy through an estate agent, their fee is built-in to the quoted price. It's not unusual to see an apartment listed with an estate agent for one price and then to see the same apartment listed privately on for 15k less. (You can select the UK flag in the top left corner for an English language version of the site.)
There is a guy in Dublin who deals with Credit Foncier in France. I think his company is called Foreign Mortgages in Dame St. and his name is Harry Saul. (I have no connection with him but did use him to help to organize a mortgage.)
One important thing to bear in mind with French mortgages is that you are obliged to take out life assurance, even if the property is an investment property. I think you are obliged to take it out with the bank themselves also but I am not 100% sure. That can add quite a bit to the cost. In fact, even though one bank was offering us a better interest rate, cheaper life assurance at another bank meant that it was cheaper to go there.
Depending on what option you choose for filing French taxes, you or may not need a French mortgage. You can choose a flat rate, something like 25% of 28% of the gross rent and then you do not need to track your expenses such as mortgage interest. Of course tax policy can always change.
I am pretty sure you can get interest-only mortgages now.
Have a look at this site for some useful information:
http://www.frenchentree.com/france-mortgage/
(I have no connection with that site.)
Consider also the rights of tenants in France. It may be difficult to evict people if you aren't happy with them. You want want to consider short-term holiday lets instead.