Ion Carbine's default fire mode is slightly better version of GM4, good for sparring and basing. Plus you can use it to PK with shotgun mode, although I'm not sure how good shotgun mode is. (Also comes with a melee)
Chem gun, well it is pretty op gun for base maybe even pk. But it costs 200 EC..If you can save to 200 ec then go for it otherwise Ion isn't a bad choice either.
Or if possible you can just a buy a GM4 with gralats and Chem with EC. GM4 will be pretty much same as Ion for spar/base.