If being told you can't put certain things in a tank is turning you off of the hobby, then a seahorse tank is certainly not for you. There are numerous restrictions on what fish, corals, and inverts can live with seahorses. Even putting captive bred seahorses with other captive bred seahorses is sometimes a no-no.
To answer your questions - the size tank will depend on what species and how many you want. Usually the minimum for full size horses is a 29 tall for 1 pair.
They don't need any special lighting, but if you want caulerpa's, you will have to have the minimum for that.
They don't need any special equipment, except perhaps a turkey baster and a feeding bowl of some sort. Some people forego these items even.
The tank must be cycled for seahorses just like any other reef tank. You cannot fill it today and add fish tomorrow.
How many in a tank will depend on what species and the size of your tank.
You do not have to use live rock and live sand, however, the benefits of these far outweigh the cost involved. If you use decorations, you will have to do more frequent water changes and have another type of filter to keep your water at it's best.
Yes, it is definitely as expensive as a reef tank. Healthy, captive bred horses are priced around 50.00-75.00 usually. Yes, you can find them cheaper, but these are sometimes wild caught horses who do not eat frozen mysis, or unhealthily kept horses who may be carrying disease. You will have to feed EVERY day. Even if your horses eat frozen, this can be expensive. If you get horses that will only eat live, you can triple that cost just in food alone.
Horses do not do well as singles. They are very social and need at least a pair to be happy. Plan when you purchase to buy as many as you will want in the future, because adding seahorses at a later date sometimes causes mysterious deaths, due to pathogens all seahorses carry.
Very few fish live well with seahorses. There are some corals that are seahorse safe, but even some of these are on an item by item factor.
This is a good place to read and learn. Other options are
www.seahorse.org and
www. sygnathid.org (the last one may be spelled wrong - I'm not a member)
Good luck - and please ask all the questions you like, especially if you are iffy on saltwater. Seahorses are high maintenance, and you should be aware of what you are getting yourself into.
