Want to expand lab network, cisco 2901 running ipbase-k9 vs cisco 2811 running advipservices-k9? more 2811s or 1721s?

Hi, apologies if this isn't the right group for this post, I'm not sure if this violates rule #1 or not... it's a lab network at my home setup for my CCNA labs and using older enterprise equipment (it's got nothing to do with video games lol), I'm just not sure where else to ask people who would know about Cisco routers?

I'm currently studying for the 200-105 exam.  I passed the 100-105 exam in February.  I had the idea of integrating my lab network with my home network so I need a router with gigabit capabilities.  My lab network is all 10/100 ethernet connections.  I've got 3 x cisco 2811 routers running 15.1-4M advipservices-k9 and 3 x cisco 3560s layer 3 switches running 12.2-44 advipservices-k9.  I'm looking for a 2901 as it's a 1U unit with gigabit ethernet and I only have 6 U left on my lab rack, 2U of which is already set aside for a NAS and a gigabit switch that my friend is selling me. 

I've been looking on ebay at refurbished and used routers as I don't have a very large budget.  All of the 2901s I've seen come with just ipbase-k9 it looks like. 

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What differences would I notice between the advipservices firmware on my lab devices and the ipbase-k9 firmware on the 2901?  I tried using the feature comparison tool on Cisco's website to compare the firmwares and had a hard time interpreting the differences.  It's evident there's definitely a number of features that are missing in the ipbase-k9 firmware vs the advipservices but I'm having trouble understanding just what they all are. It looked like it was mainly just a bunch of VoIP stuff which I'm not dealing with at all but I'm worried there were some critical features listed that I didn't see in my scan. 

I was also thinking about getting a couple more 2811s to expand my home lab. Right now I've got 4 wic-1dsu-t1-v2 cards so I've only got a very basic routing setup. I work a lot in netsim also and I'd like to be able to build more complex routing setups in my home lab that are more like the setups in netsim. I have a couple 1721s that aren't doing anything that have 12.2-4 on them I believe that came with my lab setup.

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Question number 1) Would it be a better idea to get some t1 wics and put them in the 1721s and just use what I have or should I get some more 2811s? I'm not sure on the differences between IOS 12.2-4 vs 15.1-4M in terms of features needed for CCNA studying?

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Question number 2) Can someone please break down what the differences are in terms of like categories of features I'd be missing with ipbase-k9 vs advipservices? Like if it's all just VoIP features then I won't really need them so I'd be fine with ipbase-k9. But if there's like routing stuff or other important features that'd be missing, that's a problem.

I just don't want to buy a router and then find out I can't do what I want to do with it because of firmware limitations.