Option A: spend £6K now on 4k projector with next to no content
Option B: spend £2K now on a decent 1080p projector. Put £4K aside for the future when decent 4K projectors will cost £2K and you have access to content. Means you then have £2K to spend on a decent screen or speakers.
Option B looks like to be the most sensible option. future proofing for the sake of it with new technology is usually ridiculous when you could wait and get better tech for less than half the price.
The reason why many are waiting for OLED's to drop in price before upgrading
In your opinion Sonny. OP clearly wants to spend some money and the lack of 4k content just doesn't hold up as well anymore so if the OP does get a new PJ 4k is future proofing.
Also £2750 for an older model isn't top of the line. OLED is still new tech for this market and have had plenty of issues if spending the kind of money for 75"+ you would probably want the least buggy most future proofed option.