In my opinion it is a wonderful idea to keep two budgies. They are very social parrots and do well in pairs. The only consideration is that female budgies do not typically get along with other females, whereas males get along with both males and females. Over the years I have only had one case where a male budgie did not get along with his cage mate.
Having a male and a female does not automatically mean that the female will lay eggs. They will only lay if they have a place to lay - i.e. a nesting box. If there is no nesting box then you shouldn't have a problem. A female may (very VERY rarely) lay an egg somewhere in the cage (this has never actually happened to me), and in that case you can easily remove it.
In terms of age, two young budgies will be more likely to get along (so, under 1 year for example), however I have introduced older budgies to one another many times in the past and have rarely experienced major issues - it is just something to keep in mind.
Keeping show/English budgies with small budgies is a big debate. I have always kept them together (I have 8 shows in my flock, and they have always lived with the others). Some people say that it is inadvisable due to their larger size, but I have never had any issues with shows dominating the smaller birds. If anything, shows tend to have much more mellow personalities than ordinary budgies. This is all just my experience though, so I would take this with a pinch of salt. I guess to be safe it would make sense to get a small budgie, however if you want a show then you must just consider all the pros and cons of keeping them together.
When introducing your budgies (after you have quarantined the new one for about 3-4 weeks), keep their cages next to each other and watch how they react. Then introduce them in a neutral environment outside of the cage. I would then either get a larger cage and set it up or redecorate one of the cages you already have (that is an appropriate size for two budgies) with new toys so that it is new for both of your birds when they move in together.
Good luck! While the others are right that you never know if your birds will get along, I would still take the risk as it is really lovely having more than one bird.