I hear you, it's a minefield at first.
Mine was born in early Sept. She lived in bodysuits (short sleeve, no legs) with a sleepsuit (long sleeve, covers legs, usually have "feet"). Then if we went out she'd have a cardigan and hat over that.
I'd strongly recommend buying sleepsuits with integrated scratch mitts that you can fold over - some babies will scratch themselves, and integrated ones are far easier / less likely to come off than separate mitts. Ditto for socks - get suits with feet.
Size-wise I'd buy the majority in newborn with the same quantity in 0-3 - if you leave them in the pack until needed you can always return them/re-gift if you don't need them.
Sizing... it's crazy-making. I found Next smallest, followed by F&F (Tesco) with Sainsbury's/Boots the most generous, and Gap skinny but long. I do recommend buying these from a supermarket (personally) - they are in them for five minutes and will bear the brunt of poo and vomit they may face. Get sleepsuits with buttons or zips down the front, not on the back or ones that have to go over their heads - they're faffy.
Wouldn't personally bother with trousers/tights/tops/anything else until 5/6 months (though others do) - the above is for nights and days.
Examples of sleepsuits:
www.boots.com/mini-club-3-pack-sleepsuits-10240682
(Integral scratch mitts up to 6 months)
Examples of bodysuits:
www.boots.com/mini-club-5-pack-unicorn-bodysuits-10240677
All the best with the rest of your pregnancy!