I take it you have reasons to wait until transitioning, so that it is useful to be able to pass as a man for a while?
Apparently some physicians feel it useful to start with just anti-androgens before adding oestrogen to the mix. There aren't likely to be problems for a while, but it's not good for a long-term solution, since the lack of oestrogen leads to some rather nasty effects, such as osteoporosis. In other words, if you plan to do this for more than a year or so, try to convince your doctor to prescribe oestrogen too.
As for feminising effects, they should be very mild with this kind of regime. It varies with the individual, too; there are guys on the net who agonise about growing breasts on Proscar (and I'm happy to have spent the last year growing to an A cup, still easily passable in a men's locker room), but that's listed as a relatively rare side effect. Spironolactone may well contribute, but even with that you shouldn't expect very much, except for blocking the effects of testosterone. Depending on what you want during your wait before transition, you might ask for at least a low dose of oestrogen.
The bottom line is that it's impossible to predict what exactly will happen. You can either start with oestrogen and decrease the dose if the effects start to become too noticeable, or start with just the anti-androgens and add some oestrogen if you are not satisfied with the effects. Watch what happens and keep in touch with your physician.
Nfr
