If your mechanics can handle the starting torque, and your supply can handle the starting current, DOL 100HP motor circuit breaker / contactor are available from almost any one that deals in those sorts of things and is the cheapest solution for the closed requirement "how to start a 100HP motor".
To deviate from the cheapest solution is either:
* spending money on the starter to save you money elsewhere,
* or spending money on the starter to work within the limits of your supply
* or the limits of your load
* or the requirements of your application
75kW motor running at full load 95% efficent 24h*365d is in the region of 700,000 kWh/year. (paying 8c/kWh is USD55k/year)With that you can see there is money to be made in OPEX by spending money on efficiency. If spending 5k on a VFD for example gives you 1% efficiency saving, return on investment would be one year. More expensive motor (eg ie4) is also good value. Note that at nominal speed and nominal torque, a VFD run motor usually uses more kW than a DOL motor, but below nominal torque there are significant efficiency savings (20% is typical). Then of course if you have the opportunity to decrease the speed, there is also a huge energy saving.
So your motor starting torque is maybe 2.5 to 3 times nominal torque. Can your mechanics handle that? If they can handle that, will they wear out faster? How often are you starting?
So for mechanical reasons you might want a different starter.
Depending who you buy from, star delta starter can be much cheaper or a bit more expensive to buy than a soft starter. You get more possibilities with a soft starter, sure, and you reduce jerk with a soft starter. Star delta starter you are limited to about 75% starting torque though, so you have to know if this is acceptable. Soft starter you can get to about 150% nominal torque when starting, but your starting current requirement can make this impractical. The VFD obviously is unmatched for starting torque and starting current and controllability of the same.
With starting current, you are often charged for the maximum instantaneous current your facility pulls, so using a star delta starter would reduce your starting current by 3 over DOL. A VFD can reduce it to BELOW nominal motor current, a reduction of almost 10 times Vs DOL for some motor windings.
running current of a VFD vs running current when using a DOL / star delta / soft starter is reduced. At nominal load, for a 97% efficient VFD and a .88 pf motor, your running current is reduced by roughly 9%. This can mean the difference between paying to have your supply upgraded, or for a new supply it can drop you down to a lower Ampere level (ie. Save some money.)
VFDs also come in a nice IP66 box similar to the combi starters you are used to. See for example Eaton DA1/DQ1.
A VFD is also easy to integrate into your PLC and wire. It handles every supply and every load situation. No, "oh, we didn't realise that motor needed to reverse." No "oh, we didn't realise we needed to know the amps of that motor." Or worse "you don't have the amps of that motor, you will need to figure out another clever way of solving it without purchasing any more hardware. The hardware budget is closed, and your time doesn't land on the budget of this project ..."
So if I take for example this project you have presented. I know nothing about the load, I know nothing about the supply. I know nothing about the control requirements. It is getting itself a VFD. Sure I would ask if you are running this one machine on a diesel generator with no other loads... When you say yes, really this is a whole excel spreadsheet of knowledge and not suited for this thread.
tl;dr: VFD, always, in every size motor.