I guess you might want to check Parhyale hawaiensis, this is an amphipod crustacean, for which several tools have been developed. As for the minimal necessary conditions, i think that this depends on the question you need to address. Nowadays, the capability of doing stainings, transgenesis and having a sequenced genome would be a very good start. Finally, people are working in many different arthropods nowadays, not really model systems yet, because they can approach basic questions by -omics analyses. So, again, you might want to specify, I guess, what you mean by developing new systems.

Among insects, the most promising are: Tribolium (Coleoptara, Polyphaga), parasitic hymenopterans (Hymenoptera), and Gryllus (Orthoptera: Gryllidae). Several molecular tools, have already been developed for these systems. See papers by Cassandra Extavour, Siegfried Roth and M. Grbic

It really depends on what questions you are asking. Questions at different phylogenetic levels will call for a different species of reference. Generally speaking, there is never one model that will answer your question, since every evolutionary analysis must always be comparative. Thus, you will either want to work on more than one species, or choose a well-known species, with published data, as a reference point, and then choose your study species as a species that will be the correct phylogenetic distance to serve as a comparison to answer your question.

For your second question - the minimal necessary conditions for developing a new model species is the ability to get sufficient embryos, and the ability to fix and stain them for some kind of marker. If you can't do that, forget it. The next thing you would want is a protocol for staining gene expression patterns, and a protocol for knocking down genes using RNAi or morpholinos. Once you've got the basic setup, you would want to enter the genomic age, by sequencing the transcriptome, or ideally, if you have the resources, the full genome. To really enter the Big League, you would also want to be able to do transgenesis, but this is far from trivial.

There are probably close to a hundred arthropod species set up in labs for which you can easily collect embryos and do basic staining. The number of species where there are protocols for gene expression staining and/or RNAi is probably around 25. There are currently only about 5-10 arthropods for which there are both lab techniques and a sequenced genome, but this number is set to rise very quickly over the next couple of years, with several sequencing projects on the roll. Less than 5 species have transgenesis techniques set up.

I would say that there are about 10 promising emerging lab models, covering a broad phylogenetic range. All either have a published genome, or else it is in process and will be out soon. All have at least some of the aforementioned techniques set up already. Not all techniques are feasible for all species, but your choice should be question driven rather than technique driven.

I do not known if it may be useful for you: platynereis may represent the "closer" ancestor of the well (un)known Urbilatera.

However, platynereis is not an arthropod but it is an annelid. I apologize for my answer, however I advice you to read some papers of Balavoine Guillaume. You might find something about your request. Good luck! cheers