I am not sure how important the data is, but the screen size you are talking about is typical for a standard classroom or conference room and entirely too small for a larger auditorium setup.

I realize this is for limited use, but I almost lean towards the rental/used market for this setup considering how rarely it is put into use. But, 600 people, if they are paid, even at around $20/hr., works out to their time being worth $12,000/hr while they are viewing the presentation. So, going 'inexpensive' and using a 'small' screen isn't something I would recommend if the data being presented is important and a key point of the purchase.

But, the lack of use beyond once a year is also a major factor.

Rentals offer both a larger screen which 600+ people can see and projectors which can cut through the light in the space.

The 4:3 aspect ratio died about 10 years ago with the end of the last CRT televisions and the move to HDTV. So, all new laptops and monitors sold are widescreen, most often 16:9 aspect ratio. If buying new, do NOT buy old technology, just get a new screen.

You can get two 120" screens for about $120 each, so $240.

https://www.amazon.com/Vamvo-Outdoor-Projector-Foldable-Portable/dp/B07C5FF51Q/ref=sr_1_13?keywords=projector+screen+120+inch+16%3A9&qid=1558369703&s=gateway&sr=8-13

With a 42 square foot screen you want no less than 3,500 lumens, and preferably 4,000+ lumens for optimal viewing results.

Ultra short throw projectors rarely get this bright, and are really designed around 80" interactive screens (at this point) for in-classroom use, not for presentations to a large crowd.

So, I would opt for a longer throw LCD model. Yes, you have to place it 10'+ from the screen, but it will be a more even picture and cost a good deal less than the competing ultra short throw models.

A model like this, will come close in terms of brightness and usability for that screen size...

https://epson.com/Clearance-Center/Projectors-for-Work/Pro-EX7260-Wireless-WXGA-3LCD-Projector---Refurbished/p/V11H845020-N

Getting two of everything puts you in around $1,300.

To get a better image, you can increase resolution if you want, but really, it's about adding brightness.

This model adds both brightness and resolution to the equation...
https://epson.com/Clearance-Center/Projectors-for-Work/PowerLite-1985WU-WUXGA-Wireless-3LCD-Projector---Refurbished/p/V11H619020-N

This will put you closer to $3,000, but the 4,800 lumens will make a visible on-screen difference with higher impact.

The LCD models tend to be brighter, with more accurate color, than the DLP models at the similar price point.