I don't see the benefit of improving one wheel suspension out of three. The only suspension I'd have on a side car is trailing arm, sliding pillar, or for better handling, a tilting system with bike tires so you can lean to either side with the bike and the side car moves with the lean, shifting cg inside the turn. Having the side wheel in the middle of the wheelbase is fine but can make for an exciting ride over speed bumps.
For the wheel you could have one made to your spec if you don't mind steel. Stockton wheel comes to mind.
Steering is doable but would take a considerable amount of planning for symmetry in left and right turns.
I don't know about "sporty". Actual racing sidecars have no suspension on the side wheel from what I can tell.
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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS
viewtopic.php?f=5&t=13225&p=134742Simple Spring select
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360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12