I just did 6 weeks away with a DS 7-turned 8 month old and a DD 2-turned 3 year old to extend our summer.
My husband was with us but working in the days so it was just me and the kids in the day for a lot of it.
I made sure DS had had all his vaccinations before we left. We still ended up in hospital as he had diarrhoea for over a week (guessing he drank some bathwater, our AirBnB was well-water-supplied so we avoided drinking it), so make sure you have your (and baby's) UKHIC cards and insurance (just in case it's something awful). If you don't have one already wait until baby is born and registered as its easier to apply for them both at once online (if you already have one you have to fill in a form for baby, my first form got lost at their end so it wasn't a speedy process).
Also consider the temperature of where you're going. Portugal was hot so it was tricky to take the kids out in the day (heaps of suncream, 2 hoods on the inline buggy, practically naked children). DS really wanted to eat all the sand at the beach so although it was nice to be on the beach is was far from relaxing for me.
He also became mobile while we were away so we had to buy a jumperoo and soft floor mats (tiled floor everywhere!) from fb marketplace to slightly babyproof the AirBnB, an expense we hadn't forseen.
Baby food is also different abroad so consider that if you start weaning beforehand and perhaps pack some of baby's favourite ones to take with you. I BF but if doing formula the same goes for that.
There were also no baby groups (or none that I could find, and they would have been in Portuguese anyway) so if that's your thing be prepared for a break from that. It can be a bit tiring thinking of ways to entertain a baby that age all day without help!
We also invited all our friends/family out (rented somewhere with a spare room) as it spread the parenting load on to grandparents and friends too, which was great and I would recommend! They just paid for their flight. Gives you a grown up to talk to so you don't go mad with babytalk.
Also (important one!) All airlines will give you 2 pieces of free check-in luggage for babies and children (so travel cot, car seat or buggy, the last of which you can use up to the gate then hand over at the gate). If you keep the box from your car seat, there is lots of space around it to pack other "cushioning things for your seat's protection" like clothes, nappies, blankets etc. that you might need. Same with a travel cot, cram it to increase your baggage allowance. Although you can buy nappies anywhere of course! You also get an additional piece of carry on luggage for a baby who doesn't have their own seat so do check the allowances for that. These 3 bonus luggage items weren't clear from booking but I'd read about them elsewhere so don't miss out on them!
I would do it again though 😀 we're thinking about January already 😁