If people won't be eating from a table then you need things that can be eaten easily with just a fork or fingers - rules out the slices of roast meats for me, as lovely and simple as they are.
I love Delia's chicken, tarragon and grape salad i poach chicken thigh fillets - reduces the prep, easier to chop up than a while chicken and tastier/cheaper than breast. I use shop bought mayo and you can use cheap yoghurt instead of cream.
I also love homemade quiche but that's a lot of work for that many people. Oven baked frittata would be easier. Lots of recipes if you Google.
Sticky cocktail sausages aren't just for Christmas & easy/cheap to make mounds of.
You don't mention whether you have any vegetarian / vegan guests. Roast butternut squash and chickpea salad is one of my favourites and I'm a meat eater. Have taken it to several barbecues and it always gets eaten. Just chop squash into chunks, roast, mix with tinned chickpeas and French dressing while it's still warm
You could still cook a ham, but make less go further by chopping into cubes and mixing with chopped potatoes, spring onions, sweetcorn, anything else you fancy and mix with either mayo or a dressing. Although I'd avoid having 2 dishes with the same dressing.
You've had lots of other great suggestions, you'll easily pull together a fabulous lower cost menu. Keep the salmon side for a smaller group !
My top tips:
I've find IKEA's very large ziplock bags kinder on fridge space for the kinds of salads I've suggested above than bowls, then decant on the day.
Never turn anyone down if they offer to bring anything. I'm not very good at accepting offers as I am a bit of a control freak when it comes to everything going together - so have a list of things others could bring that save fridge space whilst being reasonable to ask for eg green/Greek/tomato salad/a trifle/ a cake etc.
It would be great if you shared what you go with as I'm always on the look out for tried and tested dishes for a crowd.
Good luck!