I can buy an Android TV that has 7.0 version OS with a big discount.
Does the OS version matter?
Will I have security issues having an older version?
Will I have problems browsing web with it?
Will newer apps not have support for Android 7?
If I should be expecting such problems, then which version of androidTV would be optimal to buy in 2021-22?
Buy the best TV your budget affords. And pair it with an external streaming device. The external device is most likely more powerful than the native OS of the TV. Picture quality won’t change, but the UI experience won’t be laggy etc.
Just by the TV with the best screen and PQ and ignore whether or not it has any smart features. Then connect a device to it, like say one of the Fire Cubes on sale right now. That device will probably far exceed the performance of any built in system, and in any case you're going to eventually need to upgrade anyway far before the TV reaches end of life.
Smart TVs are todays TV/VHR combination unit--a bad purchase decision.
Will I have security issues having an older version?
As with any other unsupported software. Would you use an Android 7 phone today?
Will I have problems browsing web with it?
Android TV is not good for browsing, regardless of the version. All TV browsers suck, just each in a different way.
Will newer apps not have support for Android 7?
Probably. If not now, then later.
If I should be expecting such problems, then which version of androidTV would be optimal to buy in 2021-22?
The higher the better.
Buy a tv and add a streaming device (best Android tv streaming devices are Nvidia Shield and Chromecast with Google tv)
Just stick a Chromecast in.
Every app I've seen with a minimum requirement demand either 4.4 or newer, since apps require 6.0 or newer. I have yet to see an app that requires anything newer than that, and historically 6.0 was Android's most crucial advancement... That's the ability to turn external storage into internal storage. I think 7.0 was the first that allowed pip, and that was likely the beginning of tv os. I'm really not aware of any huge advancements past those 2 versions. If you are just gonna watch a couple video apps it will do fine and probably be worth the convenience of it already being part of the TV, but keep in mind the processor and ram will be frustratingly slow if you try to do anything else with it. Even the expensive tvs tend to put very low specs on the Android portion of the hardware.
That TV is probably over 3 years old. I would be more worried about that than Android version. Once you put the device of your choice on it the TV OS won't matter.
what device exactly... should b able to update most of the time.
While versions don't matter too much, 7 is very, very old at this point and will not be supported by many services. Quite a lot of TVs that old no longer have functioning Widevine Level 1 support too.