Look at the lot. The levelness of the lot goes into determining how long a wheelchair ramp would have to be. The requirement for a building permit for a ramp is that the ramp be 1 foot long for every inch it has to climb. A house with a slope or steps to the doorway can result in ramp that simply won't fit on the property. A ramp that has to have a turning landing adds considerably to the cost.
Porches can be a big problem. Many homes have a step or two up to a narrow porch or landing, then an extra step up to the door. This is difficult to ramp. A porch lift rather than a ramp might be necessary. Ideally the house will have two exits that can be ramped for emergencies. If a door can't be ramped the required length, a portable ramp that won't be attached to the house does not have to meet length requirements. But you have to make sure it won't be so short as to make pushing a chair up it too difficult for the caregiver or the power wheelchair motor, as well as causing the chair to tip over backwards.
Inside the house the thing to look for door widths. A simple opening without a door that is 28 inches wide should work as long as there is space for the wheelchair to approach it head on. 30 inches is the minimum needed for any doorway with a door, although off set hinges can widen the space within the door frame buy about 2 inches. Widening a door can be complicated by the need to move adjacent wall switches and outlets. Heating ducts and toilet stacks are even more of a problem. When calculating the cost of widening a doorway don't forget that the flooring or carpet will have to be replaced!
Worse than narrow doorways are turns into rooms and hallways. To determine if the turn can be made in a power wheelchair, bring along a piece of cardboard about 28" X 45". That is the footprint of a typical power wheelchair. Place it on the floor and slide it to see if it can make the turn. Fixing a tight turn is often a matter of widening the doorways, but usually it requires making it wider by several inches, not just 2 or 4.
A two story house isn't out of the question if needed for kids bedrooms, but it does need to have a downstairs bedroom and bathroom. A chair lift in the stairway for the PALS is going to be temporary. Having a great master suite upstairs where the PALS can live is absolutely not a solution either. The bedroom for the PALS has to be fairly large because a Hoyer type of lift will be necessary unless you opt for an overhead lift which is my preference. The power wheelchair will probably be spending its nights in the bedroom too. A hospital bed is nobody's preference but will be very helpful at some point. The bedroom will need to accommodate two twin beds with room to move between them even if they are pushed together at night. The hoyer lift and/or the wheelchair must fit along side the hospital bed. You will also want storage space for the added equipment and supplies. It may turn out that a separate bedroom for the spouse/caregiver gives both a better nights sleep.
And then there is the bathroom. A large bathroom or one that can be expanded is important. A roll in shower can be installed is great but have a plumber check out the bathroom before you buy the house. A roll in shower isn't just a replacement shower stall! A roll in shower is nice but not absolutely necessary. An overhead lift can lower the PALS into a tub, and with rapid progression especially, a bed bath will be easier, much faster, and more comfortable. If the toilet is in a small walled off space, the wall will most likely need to be removed if a Hoyer will be used. Hopefully the toilet stack won't be in that wall. Room to maneuver a Hoyer lift is important for using the toilet but an overhead track lift from bed to bathroom and over the toilet is ideal.
Home Inspection should include a careful inspection of the layout of the circuit box. When you start adding equipment the electrical needs in the bedroom and daytime space for the PALS will increase. BiPAP, suction machine, a tv in the bedroom, electric bed, electric heaters and blankets, special mattress, computer and peripherals and outlets with power strips to charge everything that runs on battery... It all adds up so you want to be certain that you won't be blowing a circuit everytime you turn on the microwave!
If the PALS loves to cook, that will be impossible in time regardless of any kitchen adaptations, but a big kitchen with space to oversee the cooking is good. A house with an open floor plan that allows the PALS to have space where a view of the living room and kitchen as well as a window will reduce the feeling of isolation.