Imar
generally speaking, not only relating to OFDM, the correct design flow should start from system or standard specifications to derive the architecture and the sub-specs of the receiver and/or the transmitter.
It is not a simple task, as it imlies at least 3 different skills: system, RF/analog & DSP.
If you want to limit it to the RF/analog, at least you should know, for the receiver, the required SNR at ADC.
Do you have all these info for your specific system?
OK, if you want to stay general, for an OFDM receiver, most challenging specs are the one related to Local oscillator (s) used in downconversion. If this is too high, the subcarriers of OFDM will be affected.
furthermore, being the OFDM an high peak-to-average signal, you should target linearity of your analog circuits taking into account also the peak of this signal.
Gain flatness is also a strong spec, but there are commonly used baseband algorithms that can compensate it.
What else? Many other things, but to go in detail, all the mentioned study has to be done.
A suggestion: if you are thinking to a full integrated solution, a very good book to study has been written by qizheng gu (google it), it's the only book that explain very well it for CDMA (but you can apply it to any modulation).
Mazz