in an ideal world.. but with his budget...he's better off concentrating on strictly quality not quantity. I agree a 5 piece will probably be a good idea, but 6 piece would just be adding more drums for no good reason. Most standard kits are 5 piece, so most people who would be using the house kit would be accustomed to that, if they must have shitloads of drums, a person who is that picky will more likely than not want to record their own kit, rather than the house kit.
There is some truth to this, but there is a reality that I think would counter it.
First, there are lots of standard kits that come in 4 piece offerings.
Second, if a drummer is going to the studio and is going to be playing someone elses sub $1000 kit, then all of the normal rules probably just flew out the window. What this tells me is that there is a good chance that they are not used to a standard setup. The funny things is that these days I see very few 5 piece kits with local bands. I see a lot more 4 piece or 6 or more piece kits.
In the end, here is what I have learned. I have a very nice early 80's maple Gretsch kit with 3 rack toms and 2 floor toms for clients to choose from. I also have 2 Gretsch maple snares, a chrome yamah, a little Pacific, and a Black Beauty on order. Along with this I have an assortment of 8 year old Zildijan A custom cymbals, and a set of 1966 Zildijan Avedis cymbals. This setup is MUCH nicer than a good 90% of the kits that my clients seem to play on. Even with these options, about 50% of the drummers still want to use their own shitty kits. Of that remaining 50%, about 80% of those use at least some of their own cymbals, and often even their own snare, and usually their own pedal. What this means is that I would actually sink the bulk of my money into a 5 peice, and maybe even a 6 piece shell kit before I worried about the rest. This means I get the most quality for my money and more of my clients get the most use that they will get out of it. I agree that it is important to get good cymbals. With good heads and good cymbals virtually any kit can sound pretty damned good. In the end though, clients seem to be more attached to their own cymbals, kick pedal and even snare, but seem to be more willing to not have to load their own shells up and just use mine.
I know that some of that thinking seems a bit backwards, and I even think that it is, but it also seems to be reality. Also, there are plenty of places that seem to offer a free extra tom with a kit purchase, so getting a 6 piece kit may not be any more than a 5 piece kit. It will however cover all reaonable expectations.