AM radios are a peculiar lot. Some of the very best were relative cheapies. One of the best was a very plain GE AM/FM portable that was well known among electronics sales people as being outstanding and is no longer made. Something like this Sony:http://www.universal-radio.com/catalog/spcialty/0038.html
I assume from what you say that you probably want something simple and a simple setup. Some of the best AM radios are actually car radios. They have to reject noise and be pretty selective. Of course, you have to power them with a 12v supply and build them into a cabinet for any kind of neat arrangement. Some of the old tube radios were great performers. People then listened to AM seriously and were often after distant stations.
The two antenna options are generally an outside longwire or an indoor active antenna. The Teck line seems popular:http://www.universal-radio.com/catalog/mwant.html
I'd always go for a sizable rotatable loop or at least something with some size, rather than a little mystery box. Most elaborate is something like this:http://www.kaitousa.com/KA33.htm
Notice that it makes various provisions for connecting or coupling to receivers with or without an external antenna connection and with or without its own external antenna.
I don't know that expensive tabletop AM/FM radios are any better receivers. I think their virtue is in audio quality. Even things like the Henry Kloss Model 88 and the Tivoli benefit from an external antenna.