If you buy a US system you'll just eventually end up looking for some way to get around the fact that it won't play JP HuCards. The US library is but a sliver in the great NEC scheme of things. The only titles that you'd really want US versions of would be the ones with a lot of text, and the number of text heavy HuCards is extremely small (only 3 or 4 I can think of). CDs are region free, so the killer RPGs that were released in English (Y's, Dragon Slayer, etc) will still be available to you regardless of the system's region. The only exceptions would be Neutopia, and Dungeon Explorer...and I really can't see missing those. They certainly aren't worth sacrificing Arcade Card compatability for.
All NEC consoles are reliable, but they are also all old so your milage may vary. The sound problem with original Duos is real, but its also vastly overstated.
Avoid adaptors/converters. They are a huge waste of money. They made sense back when systems were $200-600, and the converters were $15, but now they cost as much as an entire system making them totally pointless. They also make a pretty precarious set-up with the HuCard sticking way the hell out of the front of the machine just asking for someone to step on it and snap it off. Stupid looking as well.
I think the best thing going is to get a PC Engine Duo R, or RX. The only things it won't play right out of the box are US HuCards, and honestly how many of those are "must haves"? Pretty much none. US releases are almost always more rare and expensive than JP ones, and they all have crapier boxes.
I suppose everyone is different, but it seems almost certain that you'd be more interested in Japanese Arcade Card games (the use of which on US systems is a huge pain) than you would be in US HuCards.
A JP system needs a mod, in addition to a converter, in order to play US games. A US system just needs a converter to play JP games. The mod the PCE will need though is ridiculously simple.
For the record, I've been collecting Turbo/PCE stuff since about 1992, and I have both US, and JP systems so I'm not just being one of those tools that is recomending a specific avenue just because he wasted a ton of money on it.