I'm sorry to hear all that. If you sue the person you bought the house from first you may be living without heat for quite a while. I don't know anything about French law, but in America you could take out the loan to have the work done right away and THEN sue the previous owner to reimburse you for the cost. If the condition of the house was misrepresented by the people you bought it from and your lawyer is telling you there's nothing you can do about it you should probably get a second opinion from another lawyer.
On the bright side, you still have your health. A week ago you thought you had AIDS. So if this is your biggest problem you're doing OK.