Hi Tucker, and welcome.
A 4 cylinder side valve sounds like a Series 1. The motors were actually a side exhaust valve but overhead inlet valve. Either inlet-over-exhaust or in Jeep-speak an "F-head."
I have two Series 3's, but neither has the 2.25 motor, so I'm just passing on what I've heard. The 2.25 SOHC motor is just about unbreakable, with an over-engineered block and low compression it's understressed.
The diff in the back of long wheelbase Series 3's is a Salisbury, but in SWB is a standard Rover diff. There are always exceptions, so it's worth looking. The simplest thing is to compare the font and rear diffs. If they look the same, the rear is a Rover diff. If they look nothing like each other you have a Salisbury.
The weaknesses of Series 3's are firstly that they are at least 25 years old, and up to 35 years old. This will cause you more trouble than anything else: no part can be considered free of risk after 35 years.
The out-of-the-box weaknesses were an understrength gearbox (from shoehorning synchromesh into a box physically the same size as the 2a gearbox) and RUST in the chassis. The gearbox is fine provided you don't treat it as unbreakable. The transfer case is almost bulletproof. The rust issue requires checking of the chassis, and prompt repair of any issues. Since the rust is usually inside to out, tapping with a hammer is more reliable than looking for red patches.
While Series 3 chassis are more prone to rust than any other model before or after, repairs are simple and often long-lasting. One of my vehicles had large areas of the chassis cut out and replaced after less than 10 years, and has then gone the next 20 without any sign of trouble.
Welcome to the "club" and I hope you anjoy your Land Rover ownership as much as we do (which means the occasional tearing of hair along with great joy).
The most important thing to remember when driving your Land Rover is ... to wave to other Land Rover owners.
Steve
2003 Discovery 2a
In better care:
1992 Defender
1963 Series IIa Ambulance
1977 Series III Ex-Army
1988 County V8
1981 V8 Series 3 "Stage 1"
REMLR No. 215
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