This is all down to preference really...
I mainly use a Duo-R and that is definitely the best option for all round game compatibility... but you might want to look at getting the RGB mod done on it though.
But if money is tight then look at just getting an Original/Core/Core II and then when funds are available upgrade it with an IFU-30 (CD-ROM2 System) when you can.
Alternatively, buy yourself a Super Grafx now and then upgrade it with a Super CD-ROM2 System later down the line.
There are so many different options to help any budget that it does come down to how much is in your pocket now... whatever you choose it will be the best thing you ever did and you will love it. Enjoy.