You should certainly look at a diesel if you will be towing regularly. Diesels have more torque, meaning pulling power and strength. Petrol engines have more overall power but not when under a load, towing or full of gear. Like comparing a wrestler and a sprinter!
A diesel, especially the older 2003 engine models, will be noticeably slower with normal driving. Not due to age but because diesels' have developed a lot since with the newer models. But as I mentioned far better for towing. Diesel engines last much longer too, generally a 4 cylinder petrol will need a rebuild at 180 - 200,000 kms, sooner if it's had a hard life. I mention this as 2003 models you are looking at may be getting up there!
RAV 4 would be a bad choice for towing anything but a small garden trailer, the V6 model maybe, but would suffer under towing especially with 2 horses.
I have owned 2 and driven many Jeeps. Newer models are slightly better but they all suffer from build quality issues. Except the 2001 Grand Cherokee LTD model I owned, but that was built in the Graz, Austria, factory not in the US. For 2003 ish model JEEP's , buying a good one would be hit and miss and you run a strong risk of inheriting someone elses nightmare.
Landrover Defender, good for towing, but not comfortable for the trip to the shops. Discovery should be considered, Freelander the earlier noisy diesel engines should be avoided and not so good.
Possibly Mitsubishi Pajero diesel or if you can get into the price range a 2001-3 ish BMW X5 diesel.
These two may be scrutinized by other car boffins, but when you consider a neat more reliable car for a lady, with good towing, 'when it's only for 2 horses for short journeys' and suitable for shopping, look seriously at these two.
Just to add to Redogrollin who commented after me, 'this lady is in Northern Ireland. Not one of the trucks you mentioned is readily available there, and certainly not in the 2003 RAV 4 price range!' . . . . . Great info though!