Hello Lina, I'm also a newbie so have done a lot of research on choosing a camper. In the end we have gone for a Wellhouse conversion on a series 2 Alphard. Like you we were a bit concerned about bed width as I'm no slim jim but we went to check it out at the factory (who will give you a full on factory tour so you can see them being built) and to be honest I think the bed will be ok. I would recommend getting a mattress topper of some sort to even out the bumps. However we've also chosen an awning with a sleep annexe just in case we need a bit more space (Outdoor revolution cayman low, we got a deal at a show for the awning, annexe, groundsheet and carpet for the price of the awning so definitely consider visiting a show before you buy). You can sleep in the pop top but most have a weight limit of around 11st. We decided against because it's a pricy option plus if you need a pee in the night it's going to be a pain getting in and out using a ladder.
Our base vehicle is a 59k series 2, 3.5V6, 2008, and in my opinion condition trumps mileage. This is a big heavy vehicle and I think the 2.4 may be a tad underpowered as the fuel economy figures seem to be very similar for both engines which suggests the 2.4 has to work harder to move the thing. I've heard that the 2.4 hybrid may be prone to more issues but no doubt you should get more advice on that from owners of them.
We've gone for a side conversion as I didn't fancy standing outside at the back of the van with the rear door up to use the kitchen. However if you do get a side conversion I would say a pop top is pretty crucial but they are expensive so it will depend on your budget. You don't need four wheel drive unless you are going offroad.
Again this is my opinion but would suggest paying a bit more for a van conversion from a recognised dealer should give you the reassurance that the base vehicle has been carefully selected for its good condition before conversion.
Good luck
Richard