The Ambrosio rims do have a good rep, but as I said, they are more expensive than Open Pros so might stretch the budget a tad far.
I’ve not used DT road rims so can’t comment personally, but one comment is that the double-eyelet version is only available in 32h, which is a bit odd. And there are reports of the single eyelet version having problems.
But I agree, below £300 the best option must be handbuilt IMO. What price is Paul offering on Pro3 hubs, Open Pros and DT spokes? That must be a strong candidate.
Note: Paul tends to run a thicker/stronger spoke on the rear driveside (DS) to even out the tension eg. DT rev on the non-driveside and DT DB on the driveside. He also sometimes runs different nipples too, again to get good tension in the wheel, which is the main thing and why handbuilts have so many fans.