Hello MrMe
All bonsai, at some point in their lives were seedlings. So raising seedlings and "baby trees" is part of the hobby. As some of the comments about wabi sabi and visual impact, and the impression of age are concerned, young seedlings just won't be able to do this right away. But with enough time, they can. The 100 year old masterpiece did start out as a seedling - though a long time ago. For most of us raising seedlings is the nurseryman's aspect of the hobby. Many choose to deal only with older material that is well develop. Some of us really enjoy this aspect of the hobby. There is less written about the "nurseryman" aspect, but the literature is out there, just takes more digging.
My first concern is that you said they will die. I think you are right, start with "baby trees", until you can keep a batch of them going for 2 years, avoiding spending more than the price of lunch is probably a good idea. Bonsai is creating art through the use of horticulture. I would start with 10 to 20 indoor plants, when you can keep 80% alive for 2 years or more, you have the basics of horticulture, then you can graduate to a wider variety of material.
Step one - learn the horticulture. Between now and spring indoor trees are your best option. Ficus are one of the better trees for indoor growing. Others include pomegranate for a very sunny spot, grewia, eugenia, malpighia, rosemary, and many, many others.
Jade plants make good indoor bonsai, see this link
http://ibonsaiclub.forumotion.com/t16171-portulacaria-afra-spekboom-2-1-2-year-progression
yes the link is on one of the "other forums", but this forum is the most active on the web, so I am not negating this one in any way.
So first, study and learn how to grow a plant well, then you will be free to branch out and expand your skills and your collection. It is a wonderful hobby. Whether you focus on indoor bonsai, or outdoor bonsai or both, it is a great hobby.
I'm still learning more about bonsai every day, and I have been growing bonsai for over 40 years.