I'm not familiar with Ninjas, so my comments are general and may or may not apply in all cases.
A 400psi pump will have a greater water demand than a 135psi Shurflo/Flojet, so it may be necessary to instal larger diameter supply hoses and hence fittings too.
You may need an extra supply side water filter.
The higher pressure on the delivery side, made more demanding when heat is involved, will call for high specification hoses. You may have 300psi fitted at present, this will need to be changed to, say, 3000psi spec. plus fittings. The same goes for all of your solution hoses to the wand. It all depends what's fitted at present.
The components fitted to the machine may have to be moved.
As previously mentioned, your in-line heater, whether internal or external, may have to be changed.
Valves on all of your tools may need to be upgraded.
Jets may need to be changed to allow for greater water delivery. Replace brass jets regardless.
Other new components may have to be fitted or upgraded. This may apply to electrical too.
Oh, and don't forget to PAT test.
If all of this is needed, my guess would be that the price quoted from Ashby's, or any other company, would only cover the installation of the pump, costs.
Safe and happy cleaning

Ken