Yes Amigakit.com is a great Amiga store. They are even a member of this very forum if you take a look (username "amigakit.com").

Most of the regular members of classicamiga, including myself, have purchased from them and we have always been very happy with their service and their continued devotion to the Amiga community.

Another Amiga store I would like to highly recommend to you is the German store http://www.vesalia.de/ They do an equally great job and are larger in terms of content. They also support the more recent PPC systems more than Amigakit, which tends to stick more closely with the older classic Amiga range.

As for the subway, yes it is a great addition for an A1200. It is AFAIK the only USB upgrade available for the A1200, unless you did a tower conversion, then you could use a PCI USB card. However I don't think the subway is very easy to find these days, and it is quite expensive when you do find one.

An alternative to transfer files easily is via a CF card reader. For this you have two options. Amigakit sell a PCMCIA to CF card reader + the software to use it. This allows you to transfer files via CF card to the Amiga. Alternatively you can also get a CF card to IDE adapter and connect the CF card directly to the A1200's IDE port and use it like an IDE HD. It's silent, solid state, faster than an HD, and you can connect it to a PC to copy files.

Regarding software hut, I've heard that must of the items they list as being on their site are not actually available, and also that some have had trouble with them. I've never ordered from them myself so cannot say from personal experience, but I would recommend sticking with Amigakit and Vesalia if you can as I have purchased items from both many times.