Hi Manelson,
Your specs for just the GP Web Application server are killer :-)...
From what I recall in a past Convergence session about the setup for the Web App server, Microsoft says that a single server can easily handle twice the number of user sessions than a regular TS server (which is generally speaking set to accommodate about 25-30 users).. so you should be well set with your 25-35 average users login.
Here are the specs for the GP server and Web App server :
mbs.microsoft.com/.../MDGP2013_system_requirements_web_apps
mbs.microsoft.com/.../MDGP2015_System_Requirements
Though the ones for Web App are for 2013, nothing has really changed between the 2 versions.
Be aware that a single server deployment cannot be converted to a scaled configuration later on... so chose wisely. If you don't intend to ever go above your average user load, then a single-server should be fine, otherwise plan upfront for a scale-out deployement with multiple servers.