Is it dumb to buy a 3.6TDV8 instead of 4.4?...
I think the answer depends. If you can afford a a 4.4 TDV8 and get one of comparable spec/condition, then it makes no sense what so ever to go for the 3.6
The 4.4 has significantly more power and the 8 speed box makes a big difference to performance. They are genuinely rapid.
The 3.6 is a still good engine IMO and they go well too, but a reasonable rung down from what the 4.4 does. If you are seriously considering either, go and drive one of each and I'm sure you'll come to the same conclusion. If a 4.4 is realistically out of budget, then it isn't really a question as you'll only be looking at a 3.6 anyway.
However..
P38 or L322? 
This would appear to be the 'bigger' question. And ultimately it'll come down to what you are wanting from the vehicle.
Dynamically the L322 is superior to the P38. They are more powerful, faster, better mpg and generally better specced.
But there is more to it when deciding.
As you've said 3.6 TDV8 you clearly aren't looking at early BMW based L322's. I forget the exact year they went Jaguar based with Terrain Response, etc. I think it was circa 2007/8. So any L322 you are likely considering is probably 16 years old at most and maybe younger.
With the P38 the youngest are now 23 years old with the early ones knocking on the door of 30 this year.
I think this makes a difference, as an older vehicle will need to be treated more akin to running a classic car, while I'd guess most people would still think of the L322 as a modern vehicle.
Personally I prefer the look and styling of the P38. I've never really got on with the muffin top overhang the L322 has around the waste line (window line). Its one of those things that once you've seen it, you can never really un-see it.
Not saying the L322 looks bad, it doesn't. And on some AT's and lift they look pretty cool. But I do think the P38 has more classic lines and is more akin to the original. At a distance it can be quite easy to mix the outline of a p38 up with a classic.
Likewise on the inside. While the L322 is likely more plush and upmarket. I really don't like the German/BMW style of the interior. The P38 feels a lot more traditional British inside, in it design language and layout. I also think the quality of the interior of the p38 is actually very nice too.
But the real crux is how they feel. In an L322 you sit low inside with quite a high window line and a huge centre console making it feel as though you sit "
in" it. Almost cockpit like and each front seat is quite isolated from the other people in the vehicle.
The p38 on the other hand you feel like you sit '
on' it and have what feels like a low window line and large glass area. More akin to the original Range Rover. It is open, bright and airy, while most L322s feel dark, confined and more cramped (some would say cosseting).
This extends to how they drive. While the L332 is surprisingly capable off road, although relying largely on its traction systems as they will lift wheels in the air rather a lot. They are a unibody or monocoque design with independent suspension all round. This makes them drive more "
car" like on the road. To the extent that in many ways they feel rather a lot like a large heavy and tall estate car. Don't get me wrong, I'm sure an L322 is faster and flatter in the bends than a p38. But for me it is how they feel. The p38 doesn't drive like an estate car, it still feels like a "
4x4" to drive, just a very pleasant, plush one that is comfortable and quiet. And for me this is one of the biggest aspects to the car. If I want something car like... I'd just buy a car! So I very much like my 4x4s to feel and drive like 4x4s.
In fact it is the same reason I'd opt (given I had the money, which I don't) for a new Grenadier over a new Defender. The Grenadier is a lot like the p38 to drive. While the new Defender feels like a heavy BMW 5 Series estate with more body roll.
The p38 wafts like nothing else bar maybe a nicely sorted classic Range Rover. The L322 is a different sort of vehicle all together, despite the name, styling and cross over of some abilities.
The L322 is also longer, wider and heavier than a p38. Not by huge amounts, but worth noting.
So for looks, ambience, feel and passion. The p38 is a clear winner for me.
The downside is, the p38 is older, so more age related issues. And as good as the p38 was when launched, there is no denying the 2.5TD BMW engine is a weak link. Fairly low power and performance. Even when remapped, often problematic and only moderate mpg. The Rover V8 is a much nicer engine being more refined, better sounding and better performing (even against a remapped diesel). But the downside is horrendous mpg by comparison. A p38 with something like the 3.6 TDV8 engine in would be an awesome vehicle...... if only someone did such a conversion. I know a few have done the BMW M57 3.0 diesel conversion.
The other downside with the p38 is you really do need to expect to have issues with it. It is that first generation of computers and electronics to be implemented into cars such as this. And it comes with its foibles. This isn't to say an L322 is plain sailing, they can throw a major wobbly too, but on the whole the electronics and EAS seem to much more sorted.
On a p38 many things can be fixed and solved at home, which is a plus. But you really do need to expect to have a never ending stream of things that require some sort of attention (small and large). Aka running a classic car.
Ultimately it has to come down to what you want or don't want from a vehicle and what you are prepared to live with. But only you can answer this.
It majorly depends on other bidders, it is upcoming next week!
(2 stored properly in a garage)
