Would an impact event leave visible traces like impact basins, or
could the entire surface melt and reform as it is today, as I suppose
Earth did when the Moon formed? Could Venus have been a very different
planet up until 0.3 billion years ago? How could one find out, what
kind of investigation would be needed?
Certainly giant impacts were fairly common, though I don't think they were common any more as recent as 300-500 million years ago. That would have been a kind of unusual event. Mostly they occurred when the solar-system was young.
Mercury probably had one
Venus may have too - and I recommend this article as very loosely related.
The Earth had one - as you know.
and Mars too, but not quite as "giant", though still very big and it left a visible mark that spans nearly 1/2 of Mars' surface. See Here and Here.
The Moon even had one and there is evidence in the thickness of the Moon's crust and location of it's volcanic magma pools/dark spots. See Here and Here.
The point I'm trying to make with all these examples is that unless the Impact is very very large, it would leave a impact footprint of sorts. The impact has to be large enough to not only liquefy the surface but to also allow the surface enough time to reset in mostly uniform layers so you don't get 1/2 of the planet with a thicker crust than the other half. Impacts of that size are statistically improbable, within the last 300 million years. Most of the big impacts happened long before then.
Wikipedia suggests that there's other reasons for believing the global resurfacing though interior heat idea. Venus Global resurfacing event - Wikipedia - Venus lack of a magnetic field, it's influx and loss of water and D to H ratio and it's Sulfur content. Do these have had to happened through internal heat resurfacing? Now, the article says there's no proof, only some pretty good evidence, so it it possible Venus resurfacing was due to an impact? I see no reason why it wouldn't be possible.
Given Venus' Heat trapping atmosphere, higher temperature and greater gravity, which also means, greater impact velocity, at least escape velocity of about 10 km/s where as the Mars impact that covers a hemisphere was thought to be about 5 km/s and the impacting object into Mars, at least 1,600 km in radius which is nearly the size of the Moon. You might not need that big to impact Venus and create a complete resurfacing, but I wouldn't think you could go too much smaller.
I love the Venus had a moon that crashed into it idea cause that seems to work on a few levels. A giant impact that created Venus could have happened 4 and change billion years ago and created a Venusian Moon, which over time, could both slow Venus' rotation and work it's way towards Venus in the process, eventually crashing into it. It seems at least, possible, though I'm not sure what evidence could be gathered to prove it. I like your idea a lot and I think it's much more likely than this one: Earth stole the Moon from Venus.
As to Venus being very different prior to it's resurfacing. The sun was only about 3%-5% less luminous when Venus resurfaced and given Venus' proximity to the sun, I have a very hard time seeing Venus as not already in a run-away greenhouse effect even then. I love the Venus once had oceans and maybe life hypothesis, but with a CO2 rich atmosphere, it's hard for me to see it as likely. My guess is that Venus was hot and virtually without water 300 million years ago too. Unless it somehow stored a huge amount of it's CO2 underground, I have a hard time not seeing it as too hot for oceans 300-500 million years ago. Maybe, but I'm skeptical. It would be very neat to have a picture of what Venus was like prior to it's resurfacing and over the hundreds of millions and billions of years before that.
In this article it mentions that it's improbable that Venus once had a Moon based on angular momentum. A planet-Moon system maintains it's angular momentum and if the moon spirals in towards the planet, it should carry significant angular momentum with it. Such an impact should give Venus a pretty rapid rotation which would be difficult to explain given that it rotates so slowly now. Unless perhaps, the Moon was in a retrograde orbit, Moon going in one direction the planet in another and the combined angular momentum largely cancelled out, but adding more specific conditions tends to decrease the probability. I still like the hypothesis, but the article above says it's unlikely.