Not a bad mod at all, I'd suggest you rename it to the Short 23 since the specs you are using are not from the 318ti but actually the 323ti.
I'd recommend you make a second mod with minimal changes that is a 318ti with the correct engine specs, then you would be able to name that one the Short 18.

318ti engine specs:
1.8L inline 4
103 kW (140 PS; 138 hp) at 6,000 rpm
175 N⋅m (129 lb⋅ft) at 4,500 rpm
209 km/h (130 mph) Top speed

Try to drop the mass down to roughly 1255 kg / 2767 lbs with a full tank of gas for the 318ti, so make sure to lower the mass to under that number to hit that value when the card is fully topped up with fuel.

However, in regards to this car's current setup, the mass with a full tank of gas should be just under 1330 Kg / 2932 lbs, currently the mass
indicated does not include a tank of gas meaning the car is heavier than irl.

Running into issues with creating my own pages from my own png.
I get hit with 'Invalid texture TR_interior1 - Invalid file'
Properties include:
Location - C:\LFS_EDITOR\data\png
Dimensions: 512x512
Bit depth: 24
file type: png

(tbh this is probably an issue with the fact that I'm new to the mod-making scene, if so, tips would be appreciated)

Even if it's not available for multiplayer, I'd love to be able to build custom map circuits, though I'm not sure what format the maps are built in and I don't imagine they are obj files that I can construct in blender.