How much damage would there be, both physically and to the computers, memory etc, upon arrival to Gliese 445 in 40k years by radiation and interstellar debris?
As pointed out in a comment there are some pretty good answers contained in the related question about the Voyager Golden Records.
And as pointed out in another comment, the computers will be long dead. In fact the Voyager systems are expected to stop running in only a few more years when the RTG's (nuclear batteries) run out which will happen possibly as soon as 2025.
What happens after that is fairly speculative because experiments on material degradation in space normally only run for a few years. NASA has been running a program called MISSE (Materials International Space Station Experiment) for several years, launching various material samples which are attached outside of ISS for study. Below are before and after photos of a sample canister which spent four years on orbit:
MISSE 2 samples (NASA)
However most of the degradation of the MISSE samples is from atomic oxygen which is fairly prevalent in low Earth orbit, but not in deep space. Voyager 1 did however endure several years of ultraviolet and other bombardment from the Sun, however it is far enough away now that the Sun's effects have ended for the most part. For the remainder of their journeys the Voyagers will mostly be dealing with cosmic rays coming from basically all directions, as well as micrometeoroid collisions.
However very little is known about the conditions in interstellar space, which makes predictions on what Voyager will look like in 40,000 years difficult. Unless it collides with a larger meteoroid, it's likely that the structure will still be mostly intact in 40,000 years, although probably quite a bit pelted by micrometeoroids. And not just damaged on the outside, since cosmic rays can easily penetrate metal.
Of course what is interesting is thinking about what data might still be available on Voyager then. The golden record placed on each of the Voyagers is a gold-plated copper disk, the data is essentially molded into the metal and it is expected to remain intact and readable indefinitely, perhaps even for eons.
Voyager Golden Record (NASA)
However in 40,000 years the data stored in the computer's ROM will probably have been scrambled by cosmic rays. The same may happen to the data stored on the digital magnetic tapes, assuming that the tape media itself is still intact.
As a side note, in spite of how it is commonly portrayed, Voyager will not be "arriving" at Gliese 445 or even passing anywhere close to it. In 40,000 years when Voyager is about 2.3 light years from Earth it will be 1.6 light years away from Gliese 445, which itself will be 3.5 light years away from the Sun by that time, nearly as close as the Alpha Centauri system will be. You often hear this referred to as Voyager "passing" Gliese 445, but that's using the term very loosely, as this is merely the closest distance that Voyager will get to Gliese 445, which itself is currently moving in the general direction of the Sun as it heads towards its closest encounter with the Sun in 46,000 years, when Gliese 445 will "pass by" the Sun at about 3.3 light years distance.
Saying that Voyager will be passing Gliese 445 is sort of like saying when you fly due north from Las Vegas that you pass Salt Lake City. Well yes in a sense that's true, when you are 314 miles north of Las Vegas you will "pass" to the west of Salt Lake City at a distance of 171 miles.
Below is a diagram that illustrates the distance that Voyager will be from the Sun when it is closest to Gliese 445. I removed some of the extra details from the original diagram to make it easier to see Voyager and Gliese 445. You can click on the diagram below to see the details better. I also placed some red dots to make the locations easier to see, the red dot on the left is where Voyager currently is just past the heliopause. The red dot in the middle of the diagram is where Voyager will be in 40,000 years. The red dot in the section on the right at 3.45 light years is about where Gliese 445 will be in 40,000 years. Note that the diagram lists this point as the 46,000 year closest encounter with the Sun, but Gliese should be a little more to the left at 3.3 light years in 46,000 years. On the far right is the current location of Proxima Centauri and Alpha Centauri at just over 4 light years.
In the diagram it may look like Voyager will be passing even closer to the Alpha Centauri system than Gliese 445 in 40,000 years, however the diagram is just illustrating comparative distance from the Sun. Alpha Centauri is in a different part of the sky, Voyager is travelling away from Alpha Centauri in a completely different direction.