Casting Casting With Feather Lite PU

The first one mentions in the sales text that it has a filler. Try to find out what that filler is, since light fillers are often glass microbaloons and you will want to read up on those before you use them, especcially is you intend to do any kind of cutting, carving and sanding. Those things are scary, regular PU dust is like saw dust in comparison. I suspect this is why this product have the "for industrial use only"-label.
I would also study a bit further to find out more technical stuff about the resin itself. At a glance it seems it would dent fairly easily and the short pot life may not make it ideal for preassurized casting, but that depends a lot on your equipment and work flow and your needs for the finished product, so take it for what it's worth.

The clear resin sounds like a good candidate if you want clear casts. It has a long open life but that of course means that you also have to wait long before domolding. It is sensitive to humidity (well, all PU resins are, but they felt they needed to mention it specifically, so it may be more som than normal). Does your casting area have a good way to control air humidity?
If you prefer opaque casts you may want to look for another alternative.

 

I buy resin from a smaller business in your own country, so I can't help you with specific stores relevant to you. But If you know what you need; a speciallized company that spends a little less on flashy ads and childish renaming of products will always be cheaper. Look for places that sells to semiprofessionals in you area that don't have to rely on pretty packaging.
The place I buy from is in sweden with their supplier in france. Many if their products are by the brand Esprit Composite, but many of the product are unbranded, at least on the package. It usually just contain basic information on what is in the bottle so that you can easilly find the relevant MSDS/SDS. See if you can find any places like that in your aproximate area.
If you want fillers to keep costs or weight down, do some research on what is available out there and see if you can find one that fulfill your requirements. You don't need to buy resin with fillers premixed into it.
When you come across terms you don't understand, try looking them up. Wikipedia is actually not a bad place to start.

As for heat, you will typically want a low heat and low humidity enviroment. All resins are a little bit different and you will need to make you own tests, but mine preforms best at around 20 degree Celcius and 20% humidity. This needs to be maintained throughout the entire curing, so a long cure time makes it a lot more sensitive to changes. If the garage you consider working in have a door that opens directly to the outside you need to take the outside temperature and humidity into consideration. You may be able to find a way to work around this, just be aware that you may need to. In my experience, temperature mostly affects crue time, with a higher temperature making it shorter. Hight humidity (or water in contact with the casting in any form) will cause gas bubbles to form within the resin and if exteeme it can interfere with the curing.
Drafts is another thing to look out for. Not all resins are sensitive to this, but some are. So if you are having trouble with inconsistent resaults when you do you material tests, this is another thing to look at to pinpoint problem areas.

I know this sounds like a lot and even so I am only scratching the surface. Despite that, if you have the money to spend doing it right I think it's well worth it, even if you intend to get you project professionally cast eventually and here is why:
1. Basic casting skills is going to make you a better sculptor. If you understand how moldmaking and casting works you are less likely to accidentally make pieces that are impossible to cast. You will be lot easier to work with for the caster who eventually does your final casting.
2. You will be able to test out your dolls engineering in a material with the right hardness and weight and balance and adjust the joints accordingly for the final version. Even if some scultping materials comes fairly close, you'd be surprised how often a doll poses like a dream in the original material after many an hour of loving fine tuning, only to topple over when sitting, snapping it's head and refuse to hold it's arms up once it's cast.
3. It makes sense from a business perspective. If you intend to make money making dolls the main big cost is going to be in the working hours you put in to make your original sculpt. The material cost and work time for making a basic home cast will not come near that. When you send your doll off to casting you take a risk. It can get lost in shipping and there is always a small risk of it getting damaged or destroyed in the moldmaking process. If you send off your original you will be back at square one with nothing to show for it and hundreds out hours lost that you can never recover or get payed for. Not only that, you now have to start all over again on a new doll and wou won't see the money form that for the time it takes to sculpt it from scratch. Depending on the complexity and your skill level and work flow this could be anything from a few months to a year or more. If you send off a fixed up home cast on the other hand and something happens, you are only out the time it took to make the cast itself and finish it for casting, this is something that you can redo and send out again in a matter of a weak or two.

 

Oh, yes. A 3D printed doll does work a little bit like a first cast or "junk cast". It won't mimic the materials right, not in filament prints anyway, but it does give you a lot of security in that you can always print a new copy if something happens. And I have seen some pretty nice printed dolls that have been finished to look good enough to act as art dolls in their own right.

I don't have a lot of personal experience with 3D printing so I can't help you much with that, but considering the explosion of 3D sculpting we've seen here the last few years I think it's safe to assume there are plenty of people who can help you if you need tips on how to finish a print for casting and such.

Best of luck with your project!