Just to add onto this good advice, if you do go looking for a mini or portable unit, depending on where you live, I'd recommend have a browse of local free ads/classifieds as by me there's quite often small businesses like shops & offices closing down or renovating that just do a free/cheap clear out all store furniture and I've previously seen a fair few also give away various portable AC & hvac units that they used inside the business. If you can collect then you can get lots free stuff through classified ads.
On the flip side too, as long as you're not too concerned about discretion, I'd also recommend to post your own 'Wanted' ad on free ads sites asking for what you want & offering to collect, as you'd be surprised what people have hanging round in their houses that suddenly contact you! Certainly there's not always a solid guarantee on quality or cleanliness, etc, but since RRP for new units can be pricey, I thought this might be useful avenue if you looking to try & keep costs down.