you need to ask your ot to carry out an access assessment. the ot will recommend the adaptations that are necessary etc in order for your child to be able to access the setting.
be aware that some areas will already have a suitable accessible primary school (on a level) and so will be extremely reluctant to spend any more money. particulary if the building is, for example, an old victorian school etc etc.
have they decided whether to statement or not? tbh, a setting with stairs and no support is going to be a nightmare - as budgie says, she will get jostled and bumped. you need to be very clear that this is not just a health and safety issue for your child, but for all the other children too - as they will be the ones she knocks over when she falls on the steps. (dd2 has actually managed to take out the supervising TA - we are of course very proud 
)
but anyway, you need to ask for an access assessment to be carried. dd will need to work her way around the school, and the ot will come up with a report of everything that needs to be adapted, from internal and external stair rails (or ramps) to soap dispensers/ grab bars in the toilet.
as budgie says, the safest option is usually for the child with the motor impairment to go first (early) or last (late). it's ok when they are tiny (with supervision) but a right pita later when they want to be moving around with their friends.
visit a few local schools with dd, ask the lea which are accessible, and then male your mind up. if the inaccessible primary is the one you definitely want, you might have to come up with a reasonable explanation why the accessible one won't meet her needs.
are her fine motor issues going to need additional support too? (i know you haven't mentioned statementing, but it might be worth asking the q)
i'm not really sure why you are doing all the running around? where is your nursery in this? there should be a proper transition meeting etc etc. is it just because it's so early? too early? are you jumping the lea gun? speak to the area inco for early years at the lea and ask for some advice. (has she been seen by the area inco in setting at nursery? they should already have done an assessment there tbh)
she sounds similar to dd2 tbh. dd2 was independently mobile indoors for yr r but used a croc walker for playtime. they put ramps in, and double stair rails, and grab rails, and moved all the soap dispensers so she could reach them etc etc. but she was statemented with 1-1 and came with extra money. and the ht of the accessible school had already said it wasn't an appropriate setting for my dcs, so i made a point of insisting she attend the same infant school as her siblings. she doesn't have any lds. it was an old victorian school with steps everywhere.
having made it accessible for dd2 (and actually for a child who was VI - they did the painting at the same time) the school is now a much sought after placement for kids with sn... about 25% of each yr r are statemented now!