In Oz, you'll probably be looking at Metric IEC-frame motors. The US uses NEMA-frame motors and restricting yourself to these in a Metric country is likely to make life unnecessarily difficult/expensive.
IEC motor frame sizes have numbers which are the mounting surface-to-shaft centreline measurement in mm for foot-mounted motors. 90-frame motors are what most of the 2 x 72 (ish) grinders built by guys in the UK use. There are short (90S), medium (90M) and long (90L) versions. The mounting hole centres differ slightly with length on the foot-mount versions, but this is not usually a problem if building from scratch.
The Drive-End casing is usually exchangeable for either a face-mount (tapped holes in the motor end casing) or flange-mount (clearance holes in a larger-diameter flange on the motor) version. As a rule, you specify what you want and the motor supplier fits the appropriate part.
I think Oz uses 50 Hz mains (the US uses 60 Hz). 50 cycles/sec times 60 seconds gives 3000 cycles/min. Under load, there will be "slip" and the motor will run at a little under the synchronous speed.
Motors can have different pole counts. Each pair of poles requires one cycle per revolution.
A 2-pole motor at 50 Hz (3000 cycles/min) will run at a synchronous speed of 3000 RPM and a rated speed of "around" 2900 RPM.
A 4-pole motor at 50 Hz (3000 cycles/min) will run at a synchronous speed of 1500 RPM and a rated speed of "around" 1450 RPM.
A 6-pole motor at 50 Hz (3000 cycles/min) will run at a synchronous speed of 1000 RPM and a rated speed of "around" 960 RPM.
If you are running a single-phase motor, you are stuck with the plated speed of your motor.
If you are running a 3-phase motor on 3-phase mains power, you are also stuck with the plated speed of your motor.
If you are running a 3-phase motor on power provided by a Variable-Frequency Drive, you can vary the frequency and with it the motor speed.
VFDs are readily available which take a single-phase supply and output 3-phase variable-frequency power to a motor.
3-phase motors are usually able to be connected in Star (Wye) or in Delta. Below "about" 3 kW, most are able to be run on 230V Delta or 400V Star.
If you have 230V single-phase mains and use a VFD, your motor will need to be connected in 230V Delta.
Without considerably more information, it is difficult to make any firm recommendation. Some pointers:
For different speeds, you can either go for a fixed-speed motor and a multi-step pulley system, or go for a motor and VFD.
With step-pulleys, you need a shaft onto which the drive wheel is mounted, bearing for the shaft, a structure onto which to mount the bearings, a step-pulley on the shaft, a drive belt, a step-pulley on the motor shaft and a system for aligning the pulleys and tensioning the belt.
With a motor and VFD, the drive wheel can be mounted directly onto the motor shaft (24mm diameter on a 90-frame motor). This saves the cost of the separate shaft, bearings, step-pulleys and drive belt. It also allows the use of a face- or flange-mount motor, which can greatly simplify the design by eliminating many of the potential alignment issues. The cost saving on all the avoided gubbins probably won't cover the cost of a VFD, but it'll go quite a long way towards doing so.
I don't think there are many on here who would not regard the direct-drive with VFD as the better system, all else being equal.
The IEC standards effectively give standard frame sizes for different motor power outputs and pole counts. It is worth noting that there are some "compact" motors appearing that are one size down from the "standard" frame for the power output & pole count. At present, I'd avoid these because they tend not to be recommended for use with VFDs because the smaller frame cannot get the heat away as well when run at low speeds and high torque/current.
A standard 1.5 kW (2 HP), 4-pole motor or a 2.2 kW (3 HP) 2-pole motor will have a 90-frame. The rule-of-thumb for belt grinder motor power in industry is 1 HP per inch of belt width (750W per 25mm).
Standard industrial motors tend to be designed for a maximum continuous mechanical speed of 3600 RPM to cover 2-pole motors running on 60 Hz mains in those parts of the world that use it. The actual plated speed of the motor will depend on the number of poles in the windings and the mains frequency of the country in which it is sold.
Either a 2-pole motor or a 4-pole motor can therefore safely run up to 3600 RPM continuously when powered from a VFD with an output frequency of 60 Hz and 120 Hz respectively.
Pretty much any VFD will run a motor down to 10 Hz. That's 600 RPM for the 2-pole and 300 RPM for the 4-pole.
In my experience, it's nice to have the lower speed of the 4-pole available for sharpening.
If you are on a really tight budget, a single-phase, 4-pole, 80-frame motor would get you started and it could be swapped for a 3-phase, 4-pole, 80-frame motor and VFD when funds allow.
Before you design, check out belt availability. If you can easily get metric belts but not inch ones locally, it's worth making the tooling arm a little longer so that you can use 50 x 2000 belts as well. Just make sure the tooling arm doesn't hit the motor.
