The Hobart 400 is too small as it only has 400A continuous, whereas 800A continuous is recommended for the TPE 331.

Hilgard

Have you looked at Aerolithium? They will make you an oversized lithium GPU for much less than startpak. Aerolithium gets a bad rap for trying to suggest their batteries are approved for certified aircraft but in the case of GPUs, it works great for the price.

We had a flightline supervisor that insisted on starting every helicopter with a GPU. No one was going to overtemp an engine on his watch. 99% of our flights were local but then one day the guys took a helicopter on a trip and at their first fuel stop the helicopter didn't have enough power to restart. Turns out the GPU had been masking a bad battery for months. They were at a small country airport and after much searching finally found an APU, but could have easily been stuck there overnight.

If you are interested in building one, it's not difficult to build a battery cart.

A low cost, small trailer (aka Harbor Freight, Northern Tool, etc.), six 1800D 12V truck batteries and a cable will give you a 24V battery cart similar to the unit Tronair makes.

https://www.tronair.com/products/11-6602-1000

You could build it for around $1000-1500.

I had the Tronair unit that was given to me and I used it often on a C-441. I also built a smaller version with four batteries. If you need 28V, Boeing (aka Aviall) used to sell a 4V battery that was used in their Bumblebee series battery carts.

My Merlin (-10s) got fresh overhauled Nicad batteries at the A-D checks this year, starts as good on batteries as it does on GPU (about 38 seconds unless it’s cold outside). The MT props make a big difference. This is my first aircraft with Nicad batteries and I’m pretty impressed so far, plus they are serviceable.
But I do use a peak power pack 500 pretty much every time. https://peakpowerpacks.com/.