Firstly, there aren’t any grants or financial help with the installation cost.
Your correct, PV installers (generally) don’t install off-grid systems and can only do on-grid installations. EV-charging points are relatively easy to install (just like installing an outside plug), or additional electrical spur Part P required.
Off-grid PV installations were common twenty years ago, but not so much nowadays since the feed-in tariff.
The battery storage would need to be equal in size to the battery in your Ecar.
A solar management box will keep that battery topped-up until connected to your car battery.
Sorry, I can’t help any further.