This depends a lot on whether you have more time or more money. If you are going for "as awesome as possible" I would recommend masks - face paint is easier but looks commensurately less good.
The best way I know of to lay hands on cheap masks is to shop for them in the week after halloween when everyone is selling costume off cheap. Failing that if you have time rather than money, it is pretty easy to make them - you need a bunch of plaster, a head blank and modelling clay for sculpting the mask and some paint and latex. None of the stages of construction are very difficult and you will find plenty of good tutorials online, but they take a while because everything needs to dry.
If you do make a mask, be careful that eye holes are larger than you think you will need and take off the chin altogether so there is plenty of space for monster crew to breathe and for those with beards to have beards. A bit of face-paint before the masks go on will cover over the gaps more than sufficiently. Unless they are full head masks ( which are often awkward and uncomfortable ) it is worth pairing them up with hoods, which are easy and inexpensive to make.
In terms of costume the best thing you can do if you are planning to run more than one event ever ( or even if you're not ) is to make up a set of basic, cheap, machine-washable robes in dark fabric. These can be adapted in a lot of different ways and provide a reliable base-layer for different costumes. Combine those with different tabards or over-layers for quite different effects. If you need special designs or heraldry on your outfits painting it on is the easiest way to get it there- a painted shield or motif on a tabard allows you to identify one group of NPCs quickly and easily with minimal customisation to kit. Robes also have the advantage that they will cover up any jeans, Metallica t-shirts and other traditional monster-crew gear.
These cost money and time and it's worth checking you can't pick up some costume online for less than the amount you would be spending on fabric - sometimes things turn up second hand that are absolutely suitable - but if you do make your own stuff it should last you for many events, which makes for a reasonable up-front investment.
To make your monsters less human you can use foam structures under robes to create a more strange body shape as long as they are reasonably spacious. Shaping up some pipe lagging or similar over the monster's shoulders or similar will change their shape and the way the robes move with them, there's a lot you can experiment with here.
It is worth getting your materials together ahead of an event like this and with your organisation team spending a weekend making outfits and planning out other aspects of the event. Having people in one place allows you to set up a production line and it's a great opportunity to plan things out.
For customisation/decoration it is also worth doing a circuit of local thrift stores or charity shops - you can often find old fur coats, belts and other things that can be used to customise or decorate NPC outfits.
Finally, if you have a monster crew of 20 you are very lucky, but bear in mind some LARPer constants in your planning:
- Firstly we are a flaky lot, so don't be surprised if you have fewer than that. If I had 20 people absolutely guaranteed to come I would make the good kit for fifteen and have spare parts ready to make slightly half-assed kit for the other five. If I ended up with enough monsters to fill the good kit I would be happy.
- Secondly we are very flimsy- among your monster crew anticipate that you will have at least one person who is unable to take the field in any combat situations and at least one person who will be unable to wear some element of the kit, being allergic to latex masks, face paint, claustrophobic in a hood or whatever else. Consequently design your encounter so you have a few human thralls, mercenaries or similar for those who can't fit the generic costume.
- LARPers are often very enthusiastic about the hobby and happy to help out- if there are any other established groups nearby it is worth talking to them about what you have in mind, unless you have some deep and ancient rivalry, obviously. You may be able to borrow kit or monster crew in exchange for returning the favour with them.