Hey Ken,
I think you should buy the spindle that matches the work you intend to do, so first you have to decide this.
E.g. if you make only things like this,
you can see you can do everything with bits with a 8 mm shank (5/16", 0.3125"), so a spindle for ER-16 collets (bits with shank from 1 to 10 mm) would be enough, while a spindle for ER-11 collets (bits with shank from 1 to 7 mm) would be to small.
But if you are used to bits with a shank of 1/4" (6.35 mm) like you used on your hand trim router, then a spindle for ER-11 collets is fine.
Spindles with an outer diameter of 65 mm, like this one, are limited to ER-11 collets, as far as I know.
But this also means from the moment on you decide you want a 80 mm spindle, you have to choose between a spindle for ER-16 collets and one for ER-20 collets (bits with shank from 2 to 13 mm, including 12.7 mm (=1/2"). One question is, have you ever needed a bit with such a thick shank, or ever seen one? But this is not the only point to consider. As long it’s not a spindle with interchangeable tool holders (ATC, ATC spindles), the axle and the bearings are of different size on ER-16 and ER-20 spindles, which means the spindle differs in contruction.
Once the 80 mm spindle holder was out I ordered it and I came accross the question whether to buy a 2.2 kW spindle with ER-16 or with ER-20 collet. The first takes tool shanks from 1-10 mm and the latter from 1-13 mm, which would mean half-inch tools. But since at first I did not expect to use tools larger than 8 mm, I was unsure. I asked the manufacturer and he answered, the spindle with ER20 collet has larger bearings and a stiffier shaft. Larger bearings means longer life. Stiffier shaft means more precision. Also one should take into consideration that when using tools with a larger shank, the thicker the shank the stiffier the tool and so the longer the tools you can use – and thus the deeper the pockets you can mill into your workpiece. So I went for the ER20 spindle.
Further reading
1.5 kW or 2.2 kW spindle?
Regarding power, you can always choose the stronger spindle as well as the stronger VFD, as inside the VFD, you have to limit the max. power to be delivered as a mandatory setting anyway. You can use a weaker spindle on a stronger VFD by setting it accordingly, but you can even have a stronger spindle and use a VFD that only can deliver less power, and buy a stronger VFD later. What you have to consider as the most important thing regarding power, is what the supply circuit in your workshop can deliver. For this, you need to know the rated input current (in Ampères) of your VFD. For a 200–240 V VFD with single-phase input for a 2.2 kW spindle, this is usually 24 A:
(For the same power in 120 V this would be the double: 48 A).
Note that shown is the nameplate of a VFD for single-phase input (which is usual in the U.S. since you have split-phase electricity in U.S. domestic areas), which means that all the power comes over one input wire which explains why the input current is more than the double than the output current (where the current is spread over three wires, since any spindle is driven by three-phase electricity that the VFD, which in fact is an AC-to-DC-to-AC inverter, produces on the fly), and where the power is spread over three wires with 120° phase shift. On VFDs that have three-phase input, the input and the output current ratings differ less, only the VFD efficiency comes into play here.
In Europe you have 400 V three-phase electricity with at least 16 A per phase everywhere, which means you can buy a VFD with three-phase input and you can draw 11 kVA so use a 7.5 kW spindle on normal domestic supply (or the double if you have a 400 V three-phase circuit with 32 A), while in the US in domestic areas you have usually split-phase electricity with 120 V and 240 V which limits you to VFDs with single-phase input, and regarding available power on the supply circuits you can use a 1.5 kW spindle on a 120 V socket or a 2.2 kW spindle on a socket with 240 V.
Note 1: To sell you 2.2 kW spindles for U.S. 110 V, chinese VFD manufacturers simply omit the input current rating on their nameplate and in their manuals. The rating would be too high on this power (2.2 kW) for any available 120 V circuit in U.S. domestic areas.
Note 2: On chinese spindles (not only on cheap no-name, but also on Jianken spindles), the power rating is often wrong.