Some of this depends on just how much you want to spend but if you (or husband) are interested enough in the hobby it could be worth it to get it all.
There are a few things like a face shield that are safety related and that might have already been purchased but if not don't turn without one. Dust/chip extraction is another must, and a dust mask is also useful and required. Not to sure how equipped you already are but safety first.
Starting at the headstock, as has been mentioned you can use the faceplates that came with the shopsmith or you can add as a major upgrade a chuck. The shopsmith has a 5/8" spindle so a chuck will need an adapter for mounting. It is critical that you get the right adapter. For Nova chucks it is quite easy as they make an adapter and if you buy from shopsmith that will insure you have the right one. If you buy from someone else, well just make sure it comes with the shopsmith adapter. The G3 is a nice chuck but often you can find the SUPERNOVA 2 for the same or a better price and that is a better chuck. There are other brands but others can better address those. I would not buy anything less then the G3 because cheaper is cheaper but quality levels will not be there.
Now I don't know what sort of center is being used with the pen turning but again if you don't have a live center I'd think that might be a good investment and can be used with other turning as well. Shopsmith sells one (505602)again I would stick with at least this quality. Nova also sells a live center with some added features. You can see it here:
https://www.amazon.com/gp/product/B0064 ... w_c_x_17_w
Now we can talk about lathe tools all day long and if you have only the set that came with the shopsmith you will have to get at the very least a bowl gouge. The shopsmith comes with a spindle gouge, that is not the same animal. Shopsmith use to sell a bowl gouge set but I haven't seen that in a while. You have the option of going to carbide like Easy Wood Tools, or stick with conventional steel tools.
Of course to make life easier for you.... if you go with conventional tools you have to have a way to sharpen them. Shopsmith makes a couple of jigs to help with this, one that can be used on the shopsmith with the addition of a sanding disk and another that requires you to have a strip sander. The market place has a lot of grinder options and you can get carried away on this but know you will need something to sharpen on if you don't already have it.
Shopsmith also sells a set of Specialty Tool Rests, a shorter one, a 90deg, and a "S" shaped. You can find them here:
http://www.shopsmith.com/ownersite/cata ... estkit.htm
To go with that they have an upgrade to the tool rest that can be found here:
http://www.shopsmith.com/ownersite/cata ... pgrade.htm
The more ultimate version of this is the Universal Tool rest. It's a 35 pound monster and you can find that here:
http://www.shopsmith.com/ownersite/cata ... olrest.htm
You would think we would be done by now but we are not...... The standard shopsmith has a low speed of 700rpm. Sometimes even that is too fast, especially when turning. So to slow things down shopsmith sells a Speed Reducer. This gives you a low end of 100 rpm. You can see that here:
http://www.shopsmith.com/ownersite/cata ... educer.htm
The other more expensive option is to get a Powerpro headstock which gives you a low end of 250 rpm and a lot of other nice features. I expect that might be too over the top but I wanted to mention it.
Now preparing the blanks can also be an issue. It is nice to have a bandsaw to use and it is so useful for other projects as well. If you don't have one then I seriously think about adding one to the mix. Of course shopsmith sells one as do a lot of other companies.
I don't know if you know how to find the Shopsmith online catalog but it is at:
http://www.shopsmith.com/ownersite/cata ... _index.htm
It's pretty easy to spend thousands on woodworking tools so maybe a few lower level items and gift certificates might be another option. Most of us didn't buy it all at one time but got things as we needed or wanted them and it doesn't seem like such a big deal.
The only thing you really have to get is a bowl gouge(s) or Easy Wood Tools, you can see the products at:
http://www.easywoodtools.com/
(Again a reminder, do not use spindle gouges in place of a bowl gouge, (of course there is a youtube video that shows you what can happen if you don't believe me).)
The rest can be added as time goes by.
If you narrow things down a bit we might be able to give you more details at that point.
Ed