That wheelchair looks like it has an adult Corpus 3G seat. You should confirm what seat it has with the original poster before you consider moving forward with it.
If it is indeed an adult Corpus 3G seat, then it is quite adjustable for different sizes.
If you need to adjust the seat back width, you will need to purchase a new seat back (probably around $500).
If you need to adjust the seat back width or height, you will need to purchase a new seat back cushion (probably around $300).
The seat bottom can be adjusted within a certain range. If you need to adjust it beyond that range, you will need to buy a "fit kit", which includes a new seat base plate and side rails (probably around $500).
If you need to adjust the seat bottom width or length, you will need to buy a new seat cushion (probably around $300).
You will likely need a new set of batteries. I like MK batteries, and buy them online for about $500 for a set.
If the tires are worn, you will need new ones. They can cost up to $800 from Permobil, though you can buy after market tires (without rims) for about $100.
Note that if you need Roho seat cushions, you will need to replace the existing foam cushions. Roho cushions can be expensive.
Wheelchair fit is critical. Don't skimp on spending the money and time necessary to get the wheelchair fitted just to you. If you have a wheelchair shop do this for you, they will typically charge you for it.
You should confirm pricing for the above by working with your local Permobil dealer (I have been happy with NuMotion).
While this wheelchair has low mileage, it is offered at a price a bit higher than what I see locally.
For comparison, I purchased a used Permobil C500 Corpus 3G with 26 miles on it for $3,800. The C500 is much more capable than the M300. I spent about $1000 getting it to fit just right.
A critical thing to look at is the manufacture date (which is on a sticker on the wheelchair). Age matters, because Permobil has changed their wheelchairs over time while keeping the same model number.
From the pictures, it appears the wheelchair has lateral thoracic supports, lateral hip supports, and lateral thigh supports. All of those are worth having for me. The lateral thoracic supports are essential for me. These parts are stunningly expensive, though available for much less on the used market (I use some reputable vendors on ebay).