I have done a lot of embryo transfer work. You want to select a SURROGATE who is compatible size-wise with the breeding, and who has a nice temperament. The foal will have the temperament of the surrogate, not of the dam and sire (which many find surprising). You can also expect to spend about $10,000 more than the cost of "regular" breeding (also around $10, 000). You may do it for less but you need to be prepared for these costs because you won't know what they will be until you are knee deep in the process so your choice will be to bail and lose what you have invested or find a way to keep going.
What are your goals with embryo transfer? Do you feel your mare will be unable to carry the foal to term or are you not wanting to lose a season of showing but still get a foal out of her? Either are valid reasons but may have vastly different consequences on how you go about the process. Hope this helps, good luck.
As tealamutt said, embryo transfer is extremely expensive. Why did your trainer suggest it? It is done for vairious reasons however. An owner might wish to breed, but not take the mare out of the show ring. An extremely good mare who has produced excellent offspring in the past, might be a senior now and the owner might not wish to put her through a pregnancy. A mare might not have carried offspring successfully in the past, to full term.
Re. The Gypsy Horse (Vanner) owner, who has pumped out loads of identical embryo transfers each year - well in my opinion, that is pure greed. And the majority of Gypsy Horse breeders in the US, are not at all in favour of the person in Canada, using Mules as moms, when he has plenty of Gypsy mares who could carry the baby of another.
Tealamutt, I just wish not to miss out on showing and riding. The mare is quite impressive, and I'l like to get a few foals from her. She's still young, only 9, but at the peak of her career. Size wise might be difficult-she's 17.2 and BIG, but I think it would sitll be worth it...?