You have many options when you want to play PCE games.
First there are several formats of the games as:
Hu-Card,CD,SCD,ACD,SGX and there are japanese and US versions.
A "normal" PC-Engine or TG16 with no CD-ROM attached plays Hu-Cards. The white PC-Engine plays the japanese and the TG16 american cards. The TG16 can be upgraded with an adaptor to play also japanese cards. For the japanese white engine there's no such convertor, so you won't be able to play american games.
For both you can buy a CD-Rom attachment to also play CDs and SCDs when you have the right system card, which comes with the CD-ROM attachment. When you also want to play one of the 16 ACDs you'll also have to buy an Arcade Card.
When you buy a Duo (us or jap version) you get the ability to play Hu-Cards (jap cards on us Duo with converter), CDs and SCDs out of the box. The Arcade Card for the Duo is quite cheap to it won't be a problem to also buy this for $30 and you'll be also able to play ACDs.
I think to buy a Duo is the cheapest way to get the most. The reason why a TG16 lasts longer than a Duo is that it has no CD-ROM unit. The CD-ROM unit has a laser which is affective for dust and mechanical misusage.
When you want to absolutely play nearly every format you probably want to buy SuperGrafx (japanese version) with a CD-ROM attachment (some kind of rare to find) and an Arcade Card Pro. This would put you in the position to play HuCards, CD, SCD, ACD and SGX (just 5 Super Grafx cards every came out).
You can find Duos and PC-Engines at eBay or subscribe the Turbo List at
joyce.eng.yale.edu/~bt/turbo/ and post your search. You'll also find fine FAQs at the TurboList.
BTW: I've got a white PC-Engine with CD-ROM attachment and an Arcade Card Pro since 1994 and are very happy with it.