What do you need the rashguard for ?
I use one always, both summer and winther.
Althoug, we just call them "lycra's".
Only exception is the rare really hot summerdays when we can kite in shorts only, and dont use a rash guard - but very rare, as it is typically a bit colder when really windy around here, so we use a shorty mostly....
There are so many different reasons, why to wear a Rash Guard - I can think of the first 7:
1. So you dont get a sunburn if shorts only - many use long sleeved ones then.
2. So you dont get a rash from your waist harness, when in shorts only.
Every other reason is outside your wet or drysuit -
3. To avoid the wetsuit zipperstring from whipping in your head and eyes.
4. Bright colours so you can be seen in case of an emergency or rescue or just others sailing past if swimming.
5. To avoid getting your (expensive and soft) wetsuit damaged by your harness.
6. To hold your drysuit better in place.
7. To keep hold of any GPS on your upper arm, or power bars for long distances, or a mobile phone.
Would never ever consider wearing one under my wetsuits, unless I had bought a low cost suit that rashes - and then I would buy a good suit instead.
So I always use a lycra as there are so many advantages, and only one small disadvantage
It is a tiny bit colder than not wearing one (evaporation) - but in these cases where cold (typically winther), it would be very risky to ride without any bright colouring as you can not survive for long in the water, and it is very deserted and lonely in the winther - so extremely important that you can be spotted in case something goes wrong
Always uses short sleeved ones only - nothing else makes sense for me.
But in countries with more violent overhead sun, a long sleeved one could be used as sun protection of course.
When you write you live in Thailand, I assume you want to use it because of the sun ?
Sorry, I dont know how many brands there are (and I dont care - I just buy some now and then, if the size and colour is right).
But would just share some of the advantages and the reason why some of us use the lycra/rash guard always, no matter if it is hot or cold