OK if you are going to be building this and are prepared to use shield then just go for any SD card reader breakout board that is Arduino compatible. You dont need to g to the expense of a full blown shield for this.
This will do you just fine http://www.ebay.co.uk/itm//251532571285
you will need to wire up the SPI interface yourself but there are plenty of good references out there for that.
As for the Arduino board then just go for the mega. It will offer you lots of IO and has a good sized program memory and RAM. If RAM is high on your list however go for an ATMega1284P device like the BobDuino. Or get out the breadboard and build it yourself
Cheers Pete.