Get the ME add on first. You'll save money in the long run, and you'll gain a lot more experience. You can look for other multi pilots to share time with on your way to your commercial, you can also log any other complex time as well. Get the instrument instruction in a single with a little bit here and there in the multi... then maybe you can ask the examiner if you can take the IR checkride in the single, and just do a single engine NPA in the multi (so you won't be limited to multi-vfr only). There are a lot of people out there that are looking for people to share costs in multi xc's. Especially if you become an MEI, get as much multi-time as possible... and you'll already have more than the min. 15 multi pic.
If you can find a dry-rate Seneca I... you can get them to burn about 17-18gph cruise total... it won't be that bad.. just don't forget to trim for landing. The Seneca is a great airplane to get a multi in.