Hi Friend,
1) Install VCVA 5.5 (I am assuming you are talking about linux VC ), once you install VCVA, it installs even web client by default.
2) Move your host to VCVA by using web client UI (Its alternative to thick client)
On AD: If you want to join your VCVA VM to domain, you can do so by using VCVA web interface (Its not web client) :vSphere 5.5 Documentation Center
If you do not want to join to domain, you need not to take any action.
On SSO: With 5.5 SSO default database is sufficient, if you want to configure external DB , you need to configure by using VCVA web interface.
On VC database: Either default database or external. plz choose based on your requirement.
My suggestion is to check what VCVA offer by default (max host supported with default database etc) and take decision on configuring SSO and VC database. default VCVA configuration is about 3-4 clicks.
On EVC: Yes, you can use EVC with 5.0 hosts as EVC is supported from 3.5 itself.
On VUM: I am not 100% sure.
on VI client: You will have to install if required. It will not be installed by default.
Let me know if you hv any doubts.