I'm not too sure of your age, so maybe I'm half of it but i'm def twentysomething and in a professional environment.
Our receptionists on the door downstairs all must be wearing 3 items of make up. No lie - steadfast rule.
My general look is this:
Hair up, normally a bun which is sort of not high and not low as I use a doughnut and it's easy for me. If you are someone who can faff with twists. I envy you. If you do it to the side maybe think of a nice fastener for it? The slicked back look is good but if you're a bit older I'd go for the softer style. If you have straight long hair, maybe consider a fringe - sweeping to the side or full at the front. Mines full at the front which i think with hair up makes me look like a mushroom. Your hairdresser will tell you what's better. If you don't dye your hair I wouldn't start, but if you feel like a revamp then go for it!
With hair up i always go for some really nice earrings - I have small Gold pearl ones with a diamond set just above it. It just makes me look like I've bothered a bit more than just shoving my hair in a bun.
Make up - depends on you. I'm a whack it all on girl when I've got the time. But usually it's black eyeliner across the top (not too thick), this one (link should be there) is good if you're not that great with the liquid stuff across the top, some eyebrow powder, good foundation, concealer and mascara. The tinted lip balms are good as I'm shit with lipstick and gloss.
Personally, I have one or two of everything jewelry wise - earlings, necklaces, watches etc. none of which are expensive at all, but I like to rotate them so I feel different.
But if they've offered you the job they clearly like you so all of the above doesn't really matter. I'm sure you look proffessional and presentable. Most people my age spend pennies on stuff for work. Think h&m jewelry!