At your strength level I'd say 16kg kettlebell. You'll grow into it. The 20kg will be too heavy for pressing movements initially but perfect for swings within a month or two. Actually, if budget allows, get both — a 16kg and a 20kg. You'll use both forever. One more thing I'd add to any home gym — a whiteboard or chalkboard to track your workouts. Progressive overload is everything when you're trying to build muscle on GLP-1. You MUST be increasing weight or reps over time or you'll spin your wheels.

Everyone overcomplicates this. I've been on semaglutide for 14 months, lost 43 lbs, and built visible muscle definition with literally just a pull-up bar and a pair of gymnastic rings (£30 from Amazon). Ring push-ups, ring rows, ring dips, pull-ups, pistol squat progressions. Look up the r/bodyweightfitness recommended routine. It's free and it works. Spent maybe £55 total.