Congratulations thats a handsome shotgun.
Yes, you will need thin blade gunsmiths screwdrivers if you want to turn a screw on that shotgun, and the Brownells sets work well. You will likely find you need a set of the extra fine bits, but wait until the gun is in your hands to judge the need.
Cleaning it will be just cleaning the bores and standing breech after use. There is really no need to do more than pull he fore end and remove the barrels from the action to do routine cleaning.
I strongly suggest that you do not dissemble the gun beyond what is required for routine maintenance (which does not include removing the stock). Keep in mind this is a hand built gun, is unique, there are no spare parts beyond what you have made, and the further you take it apart the more likely you are to run into something unexpected.
If the locks on your IRU are like the locks on the Grulla you linked to in your post, it has four pin locks rather than five pins. Here is a marked up pic of the Grulla, with the pins circled in red:
If you decide to remove the lock plates to inspect/clean the lock works (which I confess I have done on each of the Spanish side lock shotguns Ive bought for my own use) there are some things of which you should be aware.
Your IRU appears to have the lock plate variation that is secured to the receiver using three screws. Each lock has a lock plate screw that secures that lock plate to the receiver, and these screws are the closest the standing breech. The third screw runs from the left lock to the right, and secures the two lock plates together. Here is a marked up photo of the left lock of your IRU, with the screw heads circled in red:
Before removing these screws, use a grease pencil to mark the orientation of the screw head slots to the lock plate. When re-installing the locks and lock screws, use the grease pin marks to avoid over-tightening the lock plate screws. Also, pay attention to which of the short lock plate screws secured which lock. Its best to keep using the left lock screw in the left lock, and the right screw in the right lock.
Here are photos of the left lock from an Armas Parkemy four pin side lock, presented for reference. This gun also used the three screw lock plates, and the holes in the left lock plate show the positioning of the two left lock plate screws:
Here is a photo of the exterior of the right lock from the same gun, showing the two screw holes in it:
And here is a photo of the lock work in the left lock. Both your gun and the Parkemy that these locks belong to are from the period when the use of a coil spring to tension the secondary, or safety, sear was well advanced. The reason the locks on your gun, and this one, are four screw rather than seven screw locks is largely this transition away from leaf springs to coils springs:
Id expect the lock work in your locks to look pretty much as you can see here. But these are Spanish artisanal shotguns, which means you can find almost anything.
In terms of which ammunition to use/avoid in your IRU, that depends on the gun. Spanish artisanal shotguns are purpose built guns, and each gun falls somewhere on a continuum that runs from light game gun, through medium game gun, to heavy competition gun. Where your gun falls on that continuum determines the range of proper ammunition. If you want to discuss this more, lets wait until you have the gun in hand.
