Have a think about how you'll mostly be using it - and yes defo measure car boot, doorways, storage spaces etc.
Go and have a look at the options in the shop to see what they have. Feel free to talk to the sales assistant about how you plan to use it. See what they suggest, push things around, they might even have a weighted doll you can use to test buggies, try folding and unfolding and clicking the different attachments on and off on various models. Don't agree to anything yet.
Then go home and make a list of all the criteria you've decided are important, load up a nursery site and sort the prams by price. Go through until you find the first, cheapest thing which fulfils your criteria and write down the name and price, (remember to add car seat, base, adapters, raincover etc etc cost if extra) then keep looking until you find one which is twice the price of the cheapest, or the highest you're willing to pay - there's your range of models. Then you have a longlist to research.
Don't rule out second hand. You can get some great bargains.
I recommend you choose the buggy first and look at car seat second because it narrows down your choice. A tip about car seats - for compatible ones, Maxi Cosi, Cybex, Joie, Recaro, Kiddy, Nuna and Besafe all use the same adaptors so are cross compatible. Graco, Mothercare, Cosatto, Silver Cross, Hauck and Britax can only be used with their own brand prams.