16mm or 16-55mm lens? Help!

I am getting into newborn photography and realise that my 35mm and 56mm are excellent lenses for depth of field etc, but I need a wide angle for the prop shots so I don't have to stand on something over a baby, which is extremely dangerous and not advised at all!

So I have a few options but can't decide at the moment.

1.  Use my 18-135mm for the prop shots which are only one or two in a photo shoot and sacrifice a bit of quality.  I wouldn't have any outlay but it might effect the quality of my photos and my business overall.

2.  Buy an 18mm prime secondhand - cheap as chips, but again, it may affect the quality.

3.  Buy a 16mm prime which I can use for multiple situations including events at night without a flash etc, due to the F1.4 on it.

4.  Buy a 16-55mm zoom which another newborn photographer uses in the UK and which would be a solid lens for many assignments, but I'm not a massive fan of zooms and it is a large lens this one.  It would also have a lot of uses though and would be a great lens.  However it is way more expensive.  However this is a business I'm trying to run.  I can zoom in and out in a fast way to achieve the shots I want in a hurry, plus I've often found I'm lacking a wide angle in portraiture.  I'm a bit scared though that I will stop using my 56mm as much and I just LOVE that lens!

So really option two is probably out as I might as well use my 18-135mm.  Option one is a good standby, but which of option three and four would you take?

Thanks!