Is there any network congestion or performance issue , if the office has only wireless network.

Not at all. Wireless is quite advanced these days. It depends on the use of right architecture and right hardware.

Firstly it depends upon the type of AP and the environment where AP is installed.I would suggest to check the AP model and read the technical specification and place the AP antenna accordingly.For data communication need to give10 % overlap between signals and for Voice its recommened to give30 % overlap.

It's will known that the throughput of a wireless network is much less than Wired solutions this is due to physical natural of the media it self, please note that in case you are willing to go with Wireless solution , please focus on the following parameters :
1- Bandwidth and Data Rate ( make sure to have up to300 meg theoretically ).
2- Do a wireless test surfey to select the places with you should set the AP to make sure that the power received by each AP is at least -40 dbm ( please use a software to do this function , one of the software to do this function is called Inssider .
3- make sure to select a frequency channel that is not used by other companies ( you signal should be on a free channel or on a weak channel in comparison with other company channel ).
4- Make sure to secure it well from any intrusion ( this can be done via802.x authentication which uses Radius communication method and integrate with LDAP for domain users )