Hi Cogeyl, DCC is the best way to go. You can fit decoder chips into your locos and hard wire DCC decoder chips into older ones. Cost would be around €20 per loco. A DCC controller can set you back anything from €100 to €1000. The best piece of advice I can give you is to choose your DCC controller carefully, I am planning to change mine to the NCE Power Cab 5 amp system. You can get an NCE Power Cab starter set for £136 stg and its a great system offering all the functions and easily upgradeable to the 5 amp system as your layout grows.
I found this link good http://www.dccconcepts.com/index_files/DCCtenreasonstogo.htm
The big advantages to DCC is constant power on the track, so locos can have lights on while stationary, you can run multiple locos on the same track at the same time at different speeds. You can fit sound to your locos. You can double head.
The list goes on.
Good luck with the changeover