I had a similar concern: an airplay speaker only support 11g, not 11n. I did some study and made my move and hopefully it helps by sharing what I did.
The major chip inside airplay speakers comes from one single vendor: SMSC (lately got bought by Microchip). It offers 11b/g and not 11n. No firm schedule on 11n, it takes time to make a speaker from a chip
Most 11n routers support 11b/g too. So playing music to 11g airplay speakers shall not be a problem; but it will affect your internet speed as some old routers will downgrade to 11g (which is 54Mbps, not too bad).
The new 11ac routers with multi-bands (2x2 or 3x3) can be a solution to this with ability to have separate channel for 11g airplay and other channel for 11ac internet browsing.
I did not buy any airplay speaker as it is over $100 my budget. Instead, I bought Yamaha PDX-60 dongle speaker which uses propietery RF technology which happened on sale at Amazon at that time. I did a check at Amazon store as I write and they are gone already. (relief: as I do not want to be sounded like sales :-)
Hope it helps//