Hey Tomasz,
Could you please define the scope of the processes that you are using at the moment and the ones you are having problems with which you are looking for a new solution for.
Once you have this you will probably will be able to
A) see if you cant just solve your problems with SAP
B) orient your search
(Please remember third party planning systems would still be dependent on the information being transported to them through SAP so a quicker solution might be to use SAP products to compliment your system like Vehicle Management, etc)
Best of luck,
Alex