EASA Part-FCL:
FCL.405.A MPL - Privileges
(b)The holder of an MPL may obtain the extra privileges of:
(2) a CPL(A), provided that the requirements specified in FCL.325.A are met.
(c) [IR(A) limited to co-pilot ops. Single pilot IR(A) ops only allowed if skill test IR(A) passed as single-pilot]
FCL.325.A CPL(A) - Specific conditions for MPL holders
Before exercising the privileges of a CPL(A), the holder of an MPL shall have completed in aeroplanes:
(a) 70 hours of flight time: (1) as PIC [incl. 20 hrs x-country]; ...
(b) the elements of the CPL(A) modular course as specified in paragraphs 10(a) and 11 of Appendix 3,
E to this Part; and
(c) the CPL(A) skill test, in accordance with FCL.320.
So depending on your initial MPL training, you will need at least to get 70 hrs PIC and a CPL(A) skill test. MPL programmes differ throughout Europe so check your logbook and compare to Part-FCL to see what you are missing before you can request the CPL(A) skill test. There is however no need to get a PPL, and perhaps you already logged a few PIC hours during your MPL training, but I reckon I'd still set you back at least 10k-15k.
Furthermore, even if you were to get 1500 hrs on your MPL and able to upgrade to ATPL, you would still be limited to multi-pilot operations unless you comply with CPL(A) requirements.
Goodluck!