RAILRIDE has you pretty well covered here. PS2/3 engines will run just as they would under conventional control with your setup, except you will also have access to the couplers and I believe some of the announcements... the same as you could with the correct combination of whistle/bell button presses right from a transformer.
I expect all the normal running sounds will work just fine, but you won't have access to any DCS only features. I would take a look at the instruction manual for the locomotives you are thinking about. I just typed ps3 into the search box and clicked on the first instruction manual, this one for a "Premier 4-6-2 PS-4 Steam Locomotive" http://mthtrains.com/sites/def...ction/20st19404i.pdf
Pages 8-14 of the manual show what functions can be accessed and how to access them. your TPC300 will do many of these things for you by pressing the usual buttons on the CAB1 that you would on a TMCC engine. In the case of the sample 4-6-2 locomotive it has the following features accessible via conventional control:
Marker Lights* 1 Bell, 1 Horn/Whistles
PFA (Passenger/Freight Announcements) 1 Bell, 2 Horn/Whistles
Fire the Rear Coupler 1 Bell, 3 Horn/Whistles
Fire the Front Coupler 1 Bell, 4 Horn/Whistles
Speed Control On/Off 1 Horn/Whistle, 2 Bells (from Neutral only)
Lock into a Direction 1 Horn/Whistle, 3 Bells
Reset to Factory Defaults 1 Horn/Whistle, 5 Bells (from Neutral only)
In addition squealing brakes activate automatically when you slow down quickly, and random cab announcements are played if the engine sits in neutral for a while. Also start up and shut down sounds play automatically.
I would recommend seeing how you like what you get with one MTH engine running it on your current setup. If you find it to your liking and purchase a few more, then spend the $300 on the DCS system and unlock the full feature set of your locomotives... There is no need to jump all the way in at once... especially if you are used to running conventional engines anyway.
I do not currently own any PS2 or PS3 locomotives, but others have said more than once on this forum, that the detail of these engines is typically better than comparable products by the competition, BUT the sound are better on the other brand's engines. My recommendation is to see if you can get a local hobby shop to run one for you before you pull the trigger, to see if it is to your liking.
JGL