So.. wondering about spindle and motor. Any suggestions? Something that is about the same level of performance, cost and funcionality of the rest of the machine I have described and for the use showed above: small parts in aluminium and steel.

#1: Buying a X2 casting, straight shank ER32, 2 AC bearings etc. Can be combined with either a chinese AC servo-kit or a induction motor+vfd or BLDC+BLDC driver/ESC.
Pros: Cheapest (probably), I'm familiar with ER collet system and like it, can buy proper AC bearings to start with, also familar with the casting since I have a CJ18 lathe headstock for 4th axis and it's the same thing. Can install bearings for high rpm use, will be limited by the belt drive.
Cons: Most work, might not get proper performance/straightness, If I dont get it to work it's wasted money. Might be too small/weak for future needs. RPM limited for aluminium work with 3000rpm motor and 2:1 belt drive.

#2: Buying a complete X2 head unit with R8 spindle. Can be combined with either a chinese AC servo-kit or a induction motor+vfd or BLDC+BLDC driver/ESC.
Pros: Cheap, complete unit so no work with that, R8 is widely available, can make some manual quick change system if I want
Cons: Probably cheap bearings installed - not AC? Might be too small/weak for future needs. Limited rpm range because of bearings? RPM limited for aluminium work with 3000rpm motor and 2:1 belt drive.

#3: Buying a BT30 6000rpm head from china: https://www.ebay.com/itm/Spindle-Bor...B9En:rk:2:pf:0. Can be combined with either a chinese AC servo-kit or a induction motor+vfd or BLDC+BLDC driver/ESC
Pros: Stiffness of the taper is more than I'll ever need. Quite cheap. BT30 is widely available.
Cons: More expensive than #1 and #2. Unsure about the quality of the parts and low run out? Never seen anyone actully have this. Probably cheap bearings. Limited rpm-range, but maybe better than the X2 head? RPM limited for aluminium work with 3000rpm motor and 2:1 belt drive.

#4: Chinese BT30 spindle cartridge: https://www.aliexpress.com/item/ATC-...AbTest=ae803_5

Can be combined with either a chinese AC servo-kit or a induction motor+vfd or BLDC+BLDC driver/ESC
Pros: Proper spindle cartridge. BT30 tooling. Ready for ATC/manual quick change.
Cons: Expencive. Cheap bearings? Needs some kind of housing to sit in.

#5: Chinese high speed spindle ER32, 18000rpm. Just needs a vfd to get it to run. Derated power: at 3000rpm the max output is 750W.
Pros: ER32 is simple. Motor inside spindle so no moving parts. High speed spindle for aluminium work. Simple mount.
Cons: More expensive than the X2. ER not possible with quick change. Cheap bearings? Too weak bearings for steel work?

#: Usovo SK15 ATC spindle. Can be combined with either a chinese AC servo-kit or a induction motor+vfd or BLDC+BLDC driver/ESC. Didn't find any link, but have seen this on ebay.
Pros: Cheap (about the same as X2 head if I remember correctly). High speed spindle, limited by belt drive. Ready for ATC or manual quick change.
Cons: Can't find it for sale now. SK15 is small taper and too weak for steel work. Too weak bearings for steel work?

Right now I'm favouring the X2/CJ18 casting + ER32 "diy" for cost or the chinese ER32 high speed spindle for simplicity (will be exact same setup as my router just a bit bigger spindle).

Any thoughts?

AC servo or induction motor would be 3000rpm and 2:1 belt setup so 6000rpm at the spindle.

The AC servo I looked at: https://www.aliexpress.com/item/220V...AbTest=ae803_5