Armoured limos and genestealer cults go arm in arm as far as I'm concerned. Even though limos are about as appropriate to 40k as hotpants on a space marine, I can't imagine my cult not having one or two of these in the army.
I do not think a shiny black car with bullet-proof windows would last awfully long in battle against terminators, stompas and such. To this end, I think some small concession to praticality is in order. I still want to have an identifiable car, but something you can imagine actually seeing on a battle field. Armoured car it is then. But something with at least some limo-ness about it.
The first thing vehicle proffered was the Hispano Suiza MC-36 from Minairons Miniatures. This has a great front end, I can see it fitting in nicely. Still, I thought something more angular might work better.
Next up, and already in use in some 40k armies was the Russian BA20 armoured car. This has potential. More armoured, so it actually looks like it might last a couple of minutes against las-fire. Swap out the weapons, add some 40k detailing, cult paintjob, some weathering and we could be good. Depending on the list I use they could be Chimera, rhinos or even raiders. I was thinking 'Ooo, you could do great bases for these', but 40k vehicles don't tend to have bases I see. Hmm.
I also noticed these on my travels, bases on the Rolls Royce armoured car on Moif's Wargaming Resources. These I like a lot, but I've no idea what the base models are. They look mean though.
As you can see, I spend a lot of time ferreting about for conversion parts before I go anywhere near starting a new project. Time-consuming, but it'll bear fruit eventually!