Have travelled frequently with DS from 4 mo to current age of 27mo.
IMO 3-6 is actually the dream age for travelling. So if you wanted to do longhaul that's when I'd do it. They can fit comfortably in the bassinet and you don't have to worry about meals or food for them. They aren't very mobile so don't get too cross sitting on your lap.
Travel gets more of a hassle when you have to start packing food for them, they can't fit in the bassinet and they are out of the infant carrier car seat for airport transfers.
Short haul is not too much hassle at all once they're older but I didn't enjoy long haul with DS at 13mo, 15mo, 23 mo and 24 mo. Car seat hassles, worrying about meals on long flights (they're not usually given anything if they don't occupy a seat and they don't always like your meal). They want to walk around the cabin and it's hard to fit them on the change table. Even harder is taking someone who's just potty trained.
Wherever you do go in summer consider how hot it will be with regard to little one needing to be in car seat, baby carrier, mosquitos etc So personally I'd stick to the more northern parts of Europe in July.
Also there's nothing worse than getting the flight done and waiting for the luggage (plus buggy and car seat which are often damaged!) and then facing an hour long transfer with a baby who is getting fed up.
I always check the time to hotel. But if you think you'll be using bottles you might want an apartment. Boob is obviously easier for travel.
I'd suggest a city that you're already familiar with and possibly even somewhere you speak the language should you need medical advice in a pharmacy etc