If all you can afford right now is something with 50-100w, then don't bother. It won't be much of an improvement over the guitar amp you're using.
I would agree with the 300w/1x15 target. That's a good base line for an intermediate bassist. You say you're not going to be gigging, but who knows what the future holds, right?
That amount of firepower is still small enough to be fairly portable, and "quiet" enough to use at home on your own. But also big enough to play with others in a group setting, and even play some small to medium gigs (depending on style, PA size, etc...).
In other words, if you meet or at least come close to that target, you will likely not need to buy another piece of bass amp gear ever again, unless your needs become much more than you seem to expect.
150-200w or so would probably still be fine. There are a bunch of combos out there in this range. Nothing wrong with combos per se, but if you DO end up joining a band, it's likely you would outgrow it. Wouldn't be the end of the world though. And usually adding more speakers will make a bigger difference than adding more watts. So a 150w 1x15 combo, and a 1x15 extension cab would get you about as far as 300w into a single 15. And actually, you'd be able to squeeze all the watts out of the amp into the 2 speakers, and be a little less conservative with the low end EQ.
I recommend you get the amp you need now, not the amp you envision yourself needing later.
Sorry, but this advice is actually not very good. While I would agree he shouldn't be going out and buying himself $3000 worth of bass gear and getting 1000w and 8 10s, having some excess capacity in your bass rig is ALWAYS a good thing. You always want it to be a little more than you need in a given situation.
Really, the "amp he needs now" is the guitar amp he's been using. Anything over and above that is surplus to requirements. Until/unless he starts playing in a band or at loud jams. So the whole thread is about setting him up for the near (to far) future.
That's just the nature of bass amplification (unless you are specifically going for a very "pushed" sort of sound, but I wouldn't advise that until one has a bit more experience and knows what they are doing with the gear).
You don't want to get the bare minimum know, grow into new situations and opportunities, and have instantly outgrown your equipment and need to upgrade right away.
More power is always better in the bass world, but realistic near term assessments of your needs will be what should drive your financial decisions.
Realistic, yes. But 300w and a single 15 (either in combo or head/cab form) can be had for $300-500 or less, which is really not that much more than 150w and a 10 or 12 can be had for that (that would be at all decent). Technically, I've bought a 500w head, and a 200w 1x15 for a total of $300. I mean, those were both ******** deals, but even if you switch to my 350w SWR at about 375, and add the $90 "loose" JBL 15, you're still looking at $550 for really quite a smoking small/medium capability rig that in terms of sound and build quality can stand alongside just about everything. You just have to be smart with your purchases, do research on different models, watch the market for a bit, and pounce when you see a great deal. The $225 or so 500w Trace head, was a total gamble. Sold as is out of a damaged combo. But it was a reputable seller, and the photos showed it at least powered on. So I figured I would at least get either a bangin tube preamp with goodies (12 band EQ, dual band compression, etc...) OR a 500w power section, and if I was REALLY lucky, both. And it paid off.
Plus, it's totally a buyer's market out there right now. People are just giving stuff away. Just saw a nice Genz 2x10 on ebay sell for (get ready for this...) EIGHTY NINE DOLLARS!!!!! You paid a deal like that with a decent budget head even something like a GK Backline 600 or Acoustic B200H, and you have a rig that depending on what you're into, could hold down a pro bassist for their entire career. So it really doesn't make much sense to try to spend $50 or so less and get a much less capable rig. Really in that $250-600 range, there are LIMITLESS options that in fact are REALLY GREAT. If I had the money right now I would have jumped on that GB cab in a second, even though it would mean about a 6 hour trip to get there and back.
The other thing about spending a little more to get something better, is that even if you completely give up bass a week later, you can generally get your money back out of the gear when you sell it. If you just buy cheap beginner garbage, the depreciation is brutal. For one, it doesn't have any cache, so nobody is looking specifically to buy it. Two, even if they were, it's by nature mass produced overseas and you can get it brand new for cheap already and have a warranty, return policy, etc... So there's very little incentive for someone to spend $90 on your used crappy amp, rather than buy a brand new crappy amp for $125 at GC.
