When I looked for TV last time, I can't find the one that I like that is not smart TV. So, I just bought one. Actually I was looking for just TV too. On my second thought; well, if I have smart TV, I can just switch on TV without bothering about the android box HTPC or whatever external unit. I can find one box later for more feature.

Having external one is good since the built-in one in the TV will be outdated fast. They probably update GUI and put more function. Later the unit become low on RAM/ROM and performance become sluggish. Most likely after 2 years no more upgrade and no more being bothered since new TV models come out each year nowadays 

Any external box is the best choice. Can easily replaced since they are outdated fast too. For longer stability and more flexibility, I'd go for a small PC like NUC and I'm still looking for one after a year. Waiting for better one or price going down, but every time you see new model with better feature and new price and keep waiting and waiting    Anyway, I'm in no rush because my TV is already smart TV 

Yes, buy separate external box

Normal led 4k TV + FireTV CUBE Gen2 box

get a 4k tv and android box.  the tv should last you some years but the problem is with android updates .. a box is much more flexible and cheaper option.. if slow or no update u can just update the box and not the whole package..

Just buy the TV you like within your budget. If the one you like is a smart TV, just don't bother about price difference. That was what I did 

keep in mind 2 things:

1.. priority is for the TV's hardware, picture/audio quality becos the tv is more expensive and should last u >5 years.
the trouble is the best tv for those aspects are typically "smart" or very smart or AndroidTV (ATV).
u won't find a low cost "dumb" tv that is great with picture/audio quality - that's the way TV makers work.

2.. if u r decided on a box, the next thing u need to decide is whether it is regular android or AndroidTV os.
there are some differences... some people prefer one to the other, depends on what u do with it and want u require/not require.
if u pick an ATV box, u do not want a TV with ATV too since u will have both of the same... better pick a regular android box then.

And u need search the refresh rate also. I got toshiba atv running 4k at 30hz and Sharp 630x running 4k at 60hz eventough the Sharp is way older than the Toshiba. Depends on your preference  My recommend go for Sharp 6800x if u have the budget. I got both ATV now and still I use Nvidia Shield for ease of mind