Is there a better or worse time of the year for buying seeds? I see there are many for sale but don't know when best to buy. Should they be pretty much the same or is freshest best?
If I buy some now and have a heated propogator should I be alright to get them going?

I don't know a whole lot about this but I'll add what I can. I know seed freshness is not always good. There are some species where the seed becomes more viable as it ages (maybe cryptocarpa? I can't remember.) On the other hand, some seeds lose their viability quickly as they age (for example, many carnivorous plant seeds. Some succulents also but again, I can't remember which). As far as the propagator goes, you should be fine, provided you give them the conditions they require (do you have light for them or just a heat mat?).

Also you can get some batches of seed which have very poor viability, while others are great. I guess it's best to buy from reliable sources if you can - I know mesa garden has a pretty good reputation, and I've had a pretty good germination rate so far from Vlado's seeds, not using a heat mat or lights or anything.

Maybe post what seeds you're planning on buying, and hopefully someone can give more specific information

Well, I like echinocactus, ferrocactus, tree aloes, agaves. Might also try some dracaena and baobabs.

Some seeds lose their viability over time but it seems that some other seeds need time to become more viable. Seeds of cleistocactus I picked were useless when sown straight away. I planted them 3 years later and germination rates are excellent. I have heard that sugar is a natural germination retardant. Maybe over time that sugar breaks down and allows the seeds to germinate.

Ive used heating mats, and lights. The seedlings have all been too elongated and sick looking. I plant all my seeds outside in containers now and they work that way. My seedlings grow slower at first , I dont baby them too much but they're very healthy.