I agree that more details would help narrow down suggestions. But I’ll “dump out the drawer” of my own experience with thermal paddling wear anyway and maybe some of it would be useful to you, at least in making your needs more specific for further discussion. Again, these are my personal usage schemes, developed over a few decades of dressing for conditions. Other people likely have different ones.
Used drysuits can be quite affordable. I have seen several for sale on various paddling forums in the past few months for under $300. I got my own Kokatat Goretex drysuit (worth $1200 new) used for $400.
I also have a couple of wetsuits, at least one of which I also bought used. The used one is a “3/4” (4 mm thickness in the body and thinner 3 mm in the sleeves and lower legs). It’s designed for surfing and is a fairly typical weight for such suits – quite comfortable but not ideal for paddling because it zips up the back so you are leaning against the zipper and also can’t vent it in the front as you can with a front zip.
Farmer John/Jane suits, the sleeveless ones with long legs that zip up the front, are more commonly used for kayaking and canoeing and are usually 3 mm. My personal opinion is that this is the best option for paddling and the most flexible. I have a 3 mm NRS Farmer Jane. I can wear a 1.5 mm NRS Hydroskin jacket over it in chilly conditions and then open and eventually remove the jacket as the day and/or water warms up. I also have the 1,5 mm Hydroskin pants that I can wear with the jacket in conditions where water is warmer than when I might use the Farmer Jane. I got every one of these pieces on sale at various times over the years.
There are also 0.5 mm stretchy neoprene tops and bottoms (Hydroskins are one example) that provide a little warmth but are more for protection against sunburn and abrasion in my opinion. They can be used for layering, like under shorts or a drysuit but are more like longjohns than they are useful for thermal protection on their own.
5 mm suits are warmer but substantially bulkier – more often used for SCUBA in cooler waters. They are not all that comfortable and once you get down to water temperatures under 65 anyway wind is likely to be an added factor and a drysuit is a safer option.
There are also 2/3 suits which are usually called “spring-suits”. I have a “shortie” version (sleeves above the elbows and legs above the knees) that works well for snorkeling in the Caribbean and paddling in fair weather where it is too cool for shorts and tee shirts. Even the “warm” 80 degree water in tropical areas can eventually make you feel chilled when you are immersed for a long time.
Though there are various articles and charts suggesting the best types of thermal protection based on water and air temperatures, they don’t all exactly agree. To some extent the choice is personal and people have variance in their metabolisms and level of cold and heat tolerance. For somebody starting out to build a wardrobe of thermal wear for paddling, I think a 3 mm front zip Farmer John is the most versatile core garment to start with,. If you find you need more insulation, adding a 1.5 mm or 2 mm longsleeved top or a Goretex dry top over it will add range.
The list below is sort of my approximate personal range by water temp and topside conditions. To put it in perspective – I am female, average weight (BMI of 26), have a pretty conservative metabolism and am not much bothered by cold. Except on the warmest days, I wear 5 mm neoprene hard soled booties on my feet. Also, 99% of the time I am either in a sit inside kayak or a canoe. You need more cool water protection on your lower body if you paddle a sit on top with the wetter ride that creates :
water over 80 degrees: polyester or Smartwool sportswear over a one piece bathing suit
75 to 80 degrees: 2/3 mm shortie spring suit or 0.5 mm Hydroskins (long sleeve top and bottom). Dry top in the hatch, just in case.
70 to 75 degrees: 3 mm Farmer Jane (dry top added on windy days)
65 to 70 degrees: 3/4 mm full length wetsuit (dry top added on windy days)
60 to 65 degrees: Farmer Jane plus 1.5 mm Hydroskin jacket (dry top added on windy days)
60 to 55 degrees: dry suit with 0.5 hydroskins underneath
under 55 degrees: drysuit with fleece “bunny suit” underneath
under 45 degrees: stay home