Dear Mark,
I have converted some older units because I have custom painted them but it would probably be wise if you have an older un-modifed set to check out web sites like M.B.Klein (modeltrainstuff.com) for new models before you invest in decoders etc. With the business being as competitive as it is you can often find a newer version of your favorite loco that is DCC ready or even DCC wired for a cost that is close to the retail cost of the decoder.
I have converted some older units because I have custom painted them but it would probably be wise if you have an older un-modifed set to check out web sites like M.B.Klein (modeltrainstuff.com) for new models before you invest in decoders etc. With the business being as competitive as it is you can often find a newer version of your favorite loco that is DCC ready or even DCC wired for a cost that is close to the retail cost of the decoder.