There's a lot of info on the first generation Senecas over at jaycoowners.com forums.
The GM medium-duty C5500/Kodiak is a good chassis. The diesel was a 6.6L Duramax. LLY for 2006 and beyond. Lots and lots of info on the C5500 chassis specifically, and on the Duramax diesel in the medium duty chassis over at dieselplace.com, including the Allison trans. The trans is stupid-simple to service and even has a spin-on fluid filter.
I've got an '06 Seneca on a C5500 and have been through it inside and out.
As with any vehicle that sits, your main concerns are seals and corrosion. The exterior from weather, if it sat outside, and the mechanicals from sitting full of stagnant fluids. If you don't know what you're looking for, find an RV inspector who does.
I work on mine with basic tools, there is plenty of room on these things, very easy to maintain. Mechanical parts are mostly parts-bin GM stuff and it's all widely available because a lot of it was used on other platforms and in GM's light-duty line as well. Ours was low mileage and 15 years old when we bought it, had arguably spent some time sitting. It's been a great rig for us.
The key is, with anything that old, you'll want to start fresh with filters, fluids, tires and everything consumable. If you can't do that work yourself, you'll be paying. But it's important stuff no matter what the rig. Happy to share any experiences with my Seneca if you have questions.