My preference would be to get the van into a usable condition, basic camping interior done (flat pack off the web) with working rock and roll bed etc. Make sure the van is mechanically safe and roadworthy. Give the engine some TLC (source a recommended local garage for service if needed), polish up the existing paint work and have fun this summer using it. As you use the van, you will get a better idea of the key aspects you would like to improve and what you can live with and these can be added to over time, as and when you can afford it. With a young family, reliability and safety is key....rest then follows!