Sound quality depends on the Network and the algirythum used for the compression. If good compression and no network issues you cannot tell the difference. I will use a company I know VERY well as an example for price and features.
8 port FXS(Analogue) Gateway:
2 port ISDN (Digital) Gateway: PRI/E1/T1/QSIG/CAS
Up to 48/60 channels depends on ISDN type used.
this depends on the Signalling protocol used H.323/SIP/MGCP and others. Please look at your business model and what is a requirement for you features and then pick a signaling protocol.
hope this helps.