OP, your wedding plans sound lovely. Since you're having the reception on your family's farmland, you've avoided having a wedding venue which insists on their expensive caterer, bar etc, and you're in a great position to organise your own open bar, exactly as you've suggested!
Get an extra trestle table and table cloths, and hire 2 people to stand behind it and look after glasses/pour drinks/make sure everything is OK. We did exactly that (our lovely venue organised the guy to stand behind the table at cost) and it worked perfectly. The person you hire should be willing to dress reasonably smartly, set it all up to look like a bar, ask people what they'd like etc. If you have 2 people, you'll always have at least one person behind the bar while the other one sorts out getting more wine/taking dirty glasses back to the kitchen/bringing back clean ones. It won't look or feel at all like a student party!
We ordered the wine from Tescos - the advantage being that a) they delivered to the venue, so we didn't have to transport it all and b) they actually take returns (!) - so we could order loads more than we thought we'd need, and not worry about running out. Actually, we estimated the drinks according to our own drinking habits
, and added a healthy margin... and we only used about a third of it! We just took back the rest and got a full refund. We also bought some beer (though most of our friends prefer wine), and soft drinks (we chose a couple of different fresh juices, plus the usual mixers). We asked family - and any friends who we thought to ask - if there was any particular drink they'd like to have there. A couple of people suggested particular spirits, so we got a bottle of each of those; we also got a few bottles of port, and a couple of single malts (because we knew our friends would like them
).
I think the important thing is to realise that you don't have to provide everything that a commercial bar would have. People will be quite happy with a limited selection: you just need to think about it a bit to make sure that everyone will have something they'll like. You know your friends and family, and what they like to drink - or you can easily ask them!
You will need to hire glasses - but I guess you've already thought of that, since you'll need plates etc for the grill. There are plenty of firms who hire glasses/crockery/cutlery etc - a quick internet search should give you a few options. Our venue suggested one and a half times as many glasses as people iirc. You'll also need some big containers and loads of ice to keep the wine cold (plastic bins, whatever you've got). If you have access to some big fridges to get it all cold first (so that the ice is just keeping it at temperature), then that might be best.