You don't have to install Novell Client for NetWare, because Windows XP contains an embedded client for it. To install it, you'll need to open Properties for the network connection and click Install..., then select Client and click Add... You'll have only one choice there, if the client isn't already installed, Client Service for NetWare.
It supports only IPX, so if your server runs IP only, you'll have to install the client from Novell. Make sure you configure the same IPX number for both the server and the client. On the server, it is set up in SYS;\SYSTEM\AUTOEXEC.NCF file. You can access it from the server's console by either loading INSTALL or NWCONFIG, or by editing it directly with EDIT:
LOAD EDIT SYS:\SYSTEM\AUTOEXEC.NCF
you'll find something like this in that file:
LOAD RTSSRV.LAN SLOT=3 FRAME=ETHERNET_802.2 NAME=RTSSRV_1_E82
BIND IPX RTSSRV_1_E82 NET=10
NET=10 is the IPX network number. Make sure it is the same in the properties of the client.
There is a way to configure the network on the server via, let me recall, INETCFG.NLM or a similar module (don't run it if you didn't). It moves commands that configure the network from AUTOEXEC.NCF to other initialization files. If your server is configured this way, and even if it isn't, you can find the IPX network number and other related information by issuing command CONFIG (not LOAD CONFIG, just CONFIG) from the console.
After you configure the client, the NetWare server will be visible for your Windows workstation like a Windows server. You can also map paths to it directly from Windows command line with commands similar to
net use j: \\netware42\sys /persistent:no
Guess that was the answer to questions 1 and 3.
2 Yes, basically, if your DOS programs do not access the server via NetWare APIs. If they do, install Novell client for NetWare, because the embedded client does not support most of the APIs