I don't think you are mad considering it. Nothing ventured and all that. I think the Lister is a very good choice. I'm pg from an IVF/ICSI cycle at the Lister. So if you have any specific questions, please holler.
Which doctor do you have your consult with? Reason I ask is that they all have slightly different attitudes to some aspects of treatment. Is Crohn's an autoimmune condition? If so you might be better off with one of the doctors that have an interest in the whole immune arena when it comes to reproductive failure. Just a thought.
Yes you can have your embryos screened - PGS - they can check the major chromosomes where errors are more likely to occur due to advanced maternal age (AMA). They can do a 5 panel or 7 panel screen. (13, 18, 21, x, y, 16, 22). This is done on day 3 after fertilisation whereby a cell is removed from the embryo. If you want more detail let me know. Costs £2.5K on top.
I've probably made it sound quite simple but there are lots of pros and cons. PGS does not increase the pregnancy rate but it does reduce significantly miscarriage. Lister don't do PGD which is what you would need if there was a heritable condition you didn't want to pass on. The quoted figure for your age group is that 10 - 20% of your eggs will be chromosomally normal.
I don't think you should even worry about twins at this stage.
Do you know what your day 3 FSH level is? This has a direct bearing on how well your ovaries would respond to the stimulation drugs and so how many eggs would be available for fertilisation.
One final thing - if you do go down the IVF route, don't buy your drugs from the clinic as there is an enormous mark-up on them. You could save £1k (I did) by shopping around to get the best price. Some independent chemists will do you a good deal. Lister charge £18 a vial for one of my meds and I managed to get it for £12 a vial elsewhere.
Good luck, go for it, you'll never regret trying. But you may regret not trying.