Doesnt matter how old it is it will still be good for the garden. You will likely have a lot of weeds from it though so be prepared. One way you can reduce the weed problem though would be to dig it out and compost it by turning it weekly and killing the new sprouts as they germinate. My gardens were created from feedlot clean out and even after a few years, if I let the weeds get ahead of me I have a lot of weeding to do. Good news is that the original weeds that were growing seem to have given up but now I have the 2nd generation growing.
Put in lots as early as possible and then till like crazy untill planting time.

Hello

First question, yes, you for sure can use the horse manure, how old is it? if old enough, you won't need to compost it, you can just spread it out in a inch to two inch thickness now and let it do its thing till true spring, and with the snow, you can see where you spread or you can wait till spring to do so, up to you on that one, unless it was massive horse stall it will barely help add in compost to a fraction of your acre garden, so do keep that in mind.

As for the testing, a good quality home kit will in fact do a very good job, but I do agree that with the size of the garden you are talking about, you have two choices, either take multi samples and call your country office and find out who they do the soil samples are sent to in your neck of the woods, or just plan that you will need to be starting from scratch in the garden and treat accordingly.

It does not sound like this area has been a garden before or at least for a long time- Yes? What kind of garden do you want to have, are you planning on tilling? or are you going to go low till or no till? Do you want to do old fashioned standard beds or are you going wider beds with staggered plantings, or are you planning on using cover crops and never having bare soil? When it comes to compost and feeding the soil, do you want to till in, or use honey pots or manure pits, are you going to be using mulch in the garden for weed control or are you planning on weeding?

Last but not least are you wanting to water alot or are you hoping to dryland plant as that effects what you would want to do in regards to your compost, example, if you are dry land planting, you would only do a planting stripe, lightly dig in your compost depending on plants or doubledig for others like potato's carrots, beets or other root veggies, with mulching heavily to do weeds and cover crops and mulch for the walkways to keep them under control etc.

Look forward to getting to know you better 🙂

Thank you both for the reply, it is very welcomed.

The horse manure is clean and has no weeds as it has been on the floor of the paddock for some years as is. So onward we go 🙂

Farmgal - I love you lol - I have decided we will be just flying from scratch with the garden and deal with it as I go. I had a 1/4 acre of veg growing on a previous patch of land successfully so you are right and I will just do the needs as I move on. By the way, the intent of the garden is mixed in issues I have read, seen and lived. I want to eat as much food as I can that has not been processed or government handled.