is there any issue in buying air-ap1142n-ek9 802.11A/G/N FIXED AUTO AP INT ANT and later on converting it into lightweight under the control of air-ct2504-15k9 2504 WIRELESS CONTROLLER WITH 15 AP LICENSES ?
There are no "issues" as long as:
1. You have a valid Cisco Service Contract to download the lightweight IOS;
2. You don't have more than 15 WAPs to be managed by the controller.
Are there any requirements on the SW of the WLC and the IOS of the AP?
No need. Out of the box, the 2504 will support the 1140. If you want to upgrade the 2504 to 7.2.X to, say, support IPv6, then you'll need a valid Cisco Service Contract as well.
