Renault Espace 54 plate, automatic 2.2 dci

Hi, I bought a Renault espace last year from a private seller, it worked fine for few months but recently alternator went, took it to my mechanic and he told me that there is alot of engine oil leak from engine which caused this issue, and he thinks that someone tried to do something with the engine but didn't do good job and that made few leaks.
Now, I don't have any knoweldge about cars or car engines, but he said there are quite a few leaks and it will cost you lot of money, he did mentioned 2-3 points of leaks but I only remember he mentioning 'leak from injectors too'.
I am looking for advice what to do, and in situations like these do we know how much money we are talking here, he didn't gave any quotes as he said he needs check all leaks before quoting but as part of changing the alternator he spotted few.
Also, I am not sure whether shall I get it fixed or sell it without fixing it.

Any advice, thanks