So, is it worth buying your own ac, will it be a lot more expensive?
If you've got the capital available to buy something suitable, then why not. It'll maybe save you a little money in the medium term, unless you're unlucky and get hit with some big bills.
If you want something to do as much as possible of the modular course, i.e. PPL, night qual. probably an IMCr and some serious hours building, then it's probably going to need to be something with a CofA (and associated expense). Something on a LAA permit would be cheaper, but perhaps not as useful for your particular circumstances.
Also, is the modular route PPL -> CPL -> ATPL? Does hours in a small ac count towards ATPL or do you need something bigger?
The modular route (or integrated for that matter) is PPL to CPL, often (but not quite correctly) referred to as a frozen ATPL. The CPL can, but doesn't have to, include a multi-engine rating and instrument rating. Flying hours "generally" count whatever they're in, so you could do up to 1000 in your own aircraft, whatever that happened to be (quite expensive though
). To get an ATPL issued you will need 500 hours on a multi-pilot aircraft, so that will require a type rating on "something bigger", a MCC course and an IR. For this bit you will have to have found someone willing to give you a job, unless you've got the kind of budget that will run to a MPA.
Oh, and do you need to be 23 to get a ATPL? How hard is it to find odd jobs with a CPL, like banner flying etc.? So basically, you won't get a good/serious job before you're 23 or?
You must be 21 to hold an ATPL, 18 for a CPL and 17 for a PPL.
Odd jobs with a CPL!
Nigh on impossible at the moment I would think, unless you're well connected (which I guess you're not, or you wouldn't be asking this kind of stuff on PPRuNe) or really lucky.