OP, it sounds like you're very aware that in your circumstances things may take longer. Whilst I wish you to get pg quickly and have a healthy pregnancy, this may not happen and so personally I'd go for it soon. However, do make sure you are legally protected first, whatever this means to you, whether it's marriage and making sure you share your assets snd have responsibilities coming with it, or through making sure you have enough money and your assets are protected. You should definitely live together first as well, know his family well I'd say and be transparent about the key things, money, house, career plans, location where you want to live etc. So maybe go for it when you're comfortable with it all, give it another 6-12 months to test the waters?
DH and I were engaged after 8 months, married after 1.5 years and I'm expecting a baby due on our 8th anniversary of getting together. Not through lack of trying- I was pregnant and MCd several times, we would have become parents 5 years after getting together which has been very hard. None of us have any health issues either.
One thing I would say is if you know you have a potential fertility issue, make sure you trust him enough to support you throughout. It can be a very tough journey on you both, he needs to be the right guy for it, so if you have a shadow of a doubt, don't do it for this resson alone.
I think I'd rent with him for 6 months, discuss finances, spend time with family now you can, get married in the registry office and book a wedding ahead with an option to postpone which I suspect many will allow now, and TTC. You're unlikely to have a wedding of your dreams for a while now and whilst it might be difficult to accept at 27, trust me, this isn't as important as you think now, and I'd bet many other posters will tell you the same in hindsight. It's one of those things you often look at and think in hindsight yes, it was nice, but it probably needed far less stress, effort and money and it would have been great anyway. It's not where, how much, how many people etc. that matters but marrying the right guy. Sounds trvial but it is so true. So don't get too hung up on other things. Good luck.