The biggest item you will be fighting is the size and the ambient light in the room. Your throw distance is good for the size.
The size is a bit larger than standard, but not by much.
I am assuming your current projector is a 1024x768 resolution projector, which is not very high by today's standards, but because it is likely not widescreen, you can save a bit of money on a similar resolution projector today instead of going to a widescreen model.
Here is the list of all projectors between $500 and $1,500 which will work with a 21' distance from lens to a 120" wide screen.http://www.projectorcentral.com/projectors.cfm?g=1&hide=0&st=1&r=&br=&c=&w=&ar=&db=&zr=&wt=<g=&ll=&mfg=&p=500&p=1500&wr=&dt=1.0.0&t=&pjl=0&pjw=0&pjh=0&td=21&is=120&i=w&tr=&tr2=&oop=1&sort=brt&sz=15
This list is sorted by brightness, and right at the top is a model I would consider immediately.http://www.projectorcentral.com/EIKI-EK-302X.htm
It is 5,600 lumens, likely matches your current resolution of 1024x768, and fits your throw distance.
The first model that is actually under $1,000 is this one:http://www.projectorcentral.com/Epson-PowerLite_970.htm
It's 30% less bright for that savings, and while it may be fine for portable use, it's not the one I would use.
I've gotta throw out there, that if you have this projector in use week after week, for years at a time, then a $1,000 budget is VERY low for your desires and needs. Over a five year lifespan, that works out to about $4 a week. That's not much of a budget when you consider how much use it may have and that people will need the most from it.
But - I do feel like that first model on the list would serve you well at only $1,250.
- Theater, whole house audio, and technology consultation during the build and installation process in the Washington, DC metropolitan area.