Surprise wedding

[whyohwhyisthishappening]

First time posting but long time lurker!
Been with my OH for 10ish years. Been together since we were 16. We own our own house and have a DS. We've been engaged for the past 4 or so years but we haven't got married yet mostly because of money. When we first got engaged, we were renovating our house and so finances were tight. Then our son came along and so now we don't have as much disposable income as we had previously.

We are not the most sociable of people and have family members who can be quite difficult so we've always wanted an informal and small wedding. Neither of us is after the traditional church wedding and we hate the thought of having 150 peoples' attention on us. Not putting anybody down whose dream that is, its just not us!

We are the same age and our birthdays are a month or so apart. We will be turning 30 in two years time and so I've thought this is the perfect opportunity to organise our wedding but keep it a surprise.

So I'm thinking we invite 40 or so friends and family to our joint 30th birthday party but when they turn up on the evening, its actually our wedding. We have the ceremony, buffet style food (not sausage roll style but more buffet than traditional three course meal) and a bit of dancing. This keeps the pressure off as nobody will know/be able to demand so and so gets to come/hassle us for months before to know all the details etc.

My OH is totally on board for this and we think we've found a venue. Already discussed the possibility of a surprise wedding with the venue and they like the idea and have said it'll be do-able.

My AIBU is this..
Am I being unreasonable in thinking we can pull this off?

P.S. what points come to mind straight away that make you think this might be a complete disaster?

Could you somehow arrange for it to be each other's surprise birthday- so you invite dh's friends and family and he invite yours. This introduces the necessity to be there at a certain time to 'surprise' the other one. Have one or two close friends who can be trusted to take charge on the evening and then you both arrive together and the surprise is sprung. I think you might want to allow children so people don't have childcare issues, but up to you. Otherwise I think that you will make sure that it is those people who really care about you and prioritise you who come. Also saves all the stress. Maybe arrange a weekend away with friends sometime after the wedding as alternative stag/hen do.