Good luck! 
It could be quite a good combination, two kids who are young enough to accept both of you, and grow 'into' you as a unit, and who are the same age. You are moving into your own new house, which is great, important to have neutral territory.
If it is quick, and they are young, there may be some turbulent emotions, maybe falling out between the kids, or stark realities of any differences in your parenting, as it's such a hands on age. But that could be good, to just get it all out there?
I don't know if I"m great at advice as I've not really succeeded that well with my own lot! But I do wish I'd really talked through parenting before we moved in. Even as formal as having a list of what you'd do in different scenarios. If you did it now over wine or something, a meal together, you could thrash out some potential difficulties?
Even see a counselor just to talk openly? Meet with them every 2 weeks or month to have a safe space to just thrash out any problems that came up?
Also, it will be bumpy. You won't love his kid as much as you do your own. Does your DP realize this? False expectations can really hinder progress.
Giving your kids plenty of time to be themselves, plenty of time with their own parent one to one, don't make them feel that they have to like the other child right away - or other parent. They do have to be nice and follow rules, but allow them to feel what they feel too. If bonds are to grow, they will grow slowly and not in a forced way. I.E. Not too many 'we are such a brilliant family outings' at first, go at your kids pace, not yours.
And let them decide lots about their own bedrooms, and the house too, involve them in the painting, the colors, where their toys are kept. HOpe it goes well.