Slitherine

Just for me, iv never buy new units until GC East 44 (General).
(Except few fighters in GTPG SS maybe).
Yep, only upgrade.

How often youre buy a new units in vanilla, DLC or other mods ?

It is really down to what the designer gives you. If there are a lot of capturable or given units you may need to buy few if any. I'm currently playing Allied Corps where you start with 2 recons and a tank and are given a fighter in the fourth scenario but have had to buy 2 infantry, 2 artillery, a tank and a fighter so far. By the end of the campaign I will have a core of around 35 units of which I will have bought approximately 25.

I like to buy up a variety of things and work their XP up so that if I lose something late-war, I'm not crippled with a rookie replacement. Although the game gives the player plenty of tanks, I found that I needed more:

SPAT for late war
AAA
Strategic bombers
Artillery. Lots and lots of it.

There's probably enough tanks and infantry between specials and captured units to get by without buying any extras, but usually I'll buy a few special units - eg AA, maybe AT and artillery and some strategic bombers to get the balance I prefer.

I'll sometimes buy engineers or bridgies for a scenario as they do the core job (minesweeping or river crossing) whether experienced or not. If they survive undamaged they get kept or converted but are usually disbanded again. If core slots are in short supply and prestige is OK I will convert units for a scenario and then back - Oleh Dir makes a handy bridge engineer as he can move into place and fight OK if needed.

Otherwise, straight out replacement is mainly only for when my first hero is rubbish - spotting hero on artillery or initative on a tac bomber (especially when playing Italian, US or Russians, as it's not quite so bad on a 110 or mosquito).