Cooling 25kW per rack? Easy solution
Don't do it. High density != high efficiency.
You are not running ulta low latency interconnect (Ethernet is slow) so you don't need to hit that power density which is just going to cause problems your client doesn't need.
If those boxes are really only happy up to 25C intake at full load then the cooling design is fundamentally compromised and there will be several problems;
1) Those near the top of the rack are going to run hotter whatever you do (inside the rack if not at the front intake) means you have to supply <<20C to the rack to avoid problems
2) The systems are going to be wasting power on cooling fans which will cost your client money they don't need to spend (1U cases with 36mm fans are crap for cooling a few hundred watts let alone 1kW), they could spend this money on more compute nodes instead of noisy inefficient little fans
3) The internal components are going to be running way too close to the edge of their thermal envelope, in reliability testing terms we would call this "accelerated life testing". By increasing the thermal stress your client is likely to see in 1 year how many failures the components would suffer, properly cooled, in 5 or 10 years
A decent 2U chassis (agree with you that white box is frequently as good and cheaper) would provide not just fans large enough to move air effectively and efficiently but also enough space to get sensible airflow and heatsink sizes in. You won't lose much density per rack with 2U boxes as you'll be able to get more of them in before hitting the wall on cooling.
The decent 2U chassis should also be able to run to the basic spec all the Tier 1 vendor kit is tested to, the ASHRAE TC9.9 data cent(er) environmental specification which states 28C intake is OK for years on end and 32C / 35C intake is acceptable for shorter periods. As your client will be junking the kit in a few years anyway you should be able to run up into the low 30s without problems. Of course those little 1U boxes are probably going to start dying in a big way within 18 months so look carefully at how long your client wants to keep running them for before they have to go in a skip.
If low humidity is the constraint then you don't really want to be running below a 5.5C dew point for extended periods anyway, a little bit of the hot exhaust air remixed with the cold intake after passing through a spray or evaporative humidifier works wonders for controlling minimum humidity for hardly any power cost.
The low specific humidity concern is not about the intake air humidity, it is about the relative humidity once the intake air is heated inside the server, if you are just using super dry (from cold outside) intake air then keeping the intake temp low to manage the intake RH is not going to fix the RH inside the box where the problems occur.
As for cooling 25kW (or 20kW as you could get to with 2U boxes without trouble) per rack you might consider;
1) Contained air flow, don't bother with hot / cold aisle, it doesn't work properly at those densities you will waste your life chasing hot spots and doing CFD to find out that you should have contained the air flow.
a) Doesn't matter whether you contain the hot or the cold air, given the high temperatures the GPUs run at and the hot exhaust air though you might want to contain the hot aisles and use a suspended ceiling to extract the air to outside or for cooling and recirc. Humans won't want to spend long in the exhaust of those boxes running a render job
b) In row cooling is no better than external air or perimiter CRAC units in a new build and can be rather expensive
2) Don't bother with a raised floor, at 20+kW per rack you would need a very deep plenum and a cold aisle 5-6 vented tiles wide to get enough air in without starving the systems at the bottom of the racks. Contain the hot aisles and use nice wide cold aisles, then just push the cooled air into the room and let it flow down the cold aisles. The large volume of the cold aisles will work as a plenum and feed near constant pressure air to all of the racks.
3) Overall density is not about how many kW per rack but about how many kW per square metre of total floor space, the power and cooling plant are going to get larger as the total IT kW grows and at 25kW per rack you are likely to end up wasting a lot of space due to air flow problem areas, 15-20kW per rack is likely to yield the same overall kW / m2