I personally had no problems with the 6.5 update.
I know alot of people out there say it sucked or that it was slow, but I didnt really notice anything about that untill AFTER I started tweaking and modding the phone.
If you keep most of everything stock, you wont have any issues. Especially if you end up dissabling sense.
I would hold off on flashing a custom rom untill youve had the phone for awhile, once you unlock the device and start flashing, if you get carried away then you can easily ruin the phone befor you know what has happened.
Especially if your not sure what is really going on lol.
Do extensive research befor flashing, ask any question you can think of asking, were here to help
I ran into an issue myself cause I flashed a rom not too long ago (it was a MaxSense based rom) and it pretty much ruined my phone. That roms homescreen took WAAAAAYYYY to much of the processing power of the phone just to run the home screen. It was always freezing and rebooting that it was just a mess. I ended up flashing away from it within a week, but the damage was already done, my phone is very laggy and slow since then. Even running just titanium (striped homescreen without sense that was already mensioned)
I am personally running Energy referance 6.5.3
It is ecencially exactly the same as 6.5 but has a differnet interface and is a bit more finger friendly/ tweekable.
It comes with a hotspot built in, I use it everyday.
And I compleatly understand where your comming from. I too am on 30 dollar SERO
And a 4g device just about doubles the monthly costs.
I dont even get 4g anywhere short of an hour away from here in any direction