Most online stores for running shoes have pretty good return policies, so i’d just buy from those that allow returns. But i’ve only had to use returns once or twice between my many orders. Tbh i’ve only ever bought my first pair locally, felt ripped off paying 140€ for a pair of asics gt-2000 and ever since ordered online without issues.

You want them to be comfortable and with roughly 1 thumb width of space in the front. Also lacing matters a lot. I’ve had shoes hurt after a few km running, because i laced them too tightly. Which was fine at the start, but during your run your feet will swell up a bit (that is also what the extra space in front helps with).

As said for me the thing that works is to google a size chart for the model and not go by EU sizes, but cm (or JPN sizing, which equals cm). Those are very consistent between brands. For example my “main” brand is probably saucony where i have a EU46, which correlates to 29,5cm. I had my Brooks Hyperion tempo in EU45,5, which for brooks is 29,5cm. And i had 3 pairs of puma so far (2 velocity, 1 liberate nitro) all in 46 that in reality are actually a bit long, which tracks with the fact that for puma EU46 is 30cm. So i know i ideally want 29,5cm with 30cm being fine (but anything shorter doesnt work).

Beyond that it is just minor fit differences, e.g. altra shoes have wide toe boxes, nike are said to run a bit narrow and so on. Usually you can learn these from a decent review, if they mention it.

That said for this to work you need to either know your shoe size at least once. Either from your old shoes, trying some in store, or you estimate with the guides on online sites (and return if they end up the wrong size).

As for which type of shoes to buy, unless you have a rotation of multiple shoes you’ll probably just want a decent and versatile daily trainer, which basically every brand offers.